0042-6989/92$5.00+ 0.00 Copyright 0 1992Pergamon Press Ltd
VisionRes. Vol. 32, No. 11, Pp. 2087-2092,1992 Printed in Great Britain. All rights reserved
Reaction Time Distributions and Their Relationship to the Transient/Sustained Nature of the Neural Discharge STEVEN
H. SCHWARTZ*
Received 29 January 1992; in revised form 30 April 1992
Reaction time distributions (RTDs) were determined in response to near-threshold increments of long duration. Stimulus parameters were selected to isolate the chromatic and achromatic systems. The RTDs for the achromatic system peak sooner and are more narrow than those obtained for the chromatic system. These results are analyzed in terms of the neural discharge pattern of parvo and magno pathways. Color vision Reaction times Magno system
Temporal vision
Chromatic system Achromatic system
It is widely assumed that visual information in the primate is transmitted along two major retina-cortical pathways, referred to as the magno (M) and parvo (P) pathways. These pathways are anatomically distinct, up through at least visual area 2 (Livingstone & Hubel, 1987). The neurons which constitute these pathways show distinct morphologies and different neurophysiological characteristics (De Monasterio & Gouras, 1975; Dreher, Fukada & Rodieck, 1976; Schiller & Malpeli, 1978). P cells, located in the dorsal four layers of the dorsal lateral geniculate nucleus (dLGN) are smaller than M cells, which are confined to the two ventral most layers. P cells show wavelength opponency, slow conduction velocities and sustained responses to a step input. By contrast, M cells show weak, if any, color opponency, fast conduction velocities and relatively transient responses to a step input. It has been suggested that the P pathway codes hue (or color) information and the M pathway transmits luminance (or brightness) information (Shapley, 1990). Given the physiological characteristics of the M and P pathways, one can put forward the hypothesis that stimuli detected by the P pathway will be detected with a longer visual latency than those detected by the M pathway. A number of psychophysical studies have demonstrated that stimuli detected by the putative psychophysical counterpart of the M pathway, the achromatic system, are seen with a shorter visual latency than those detected by the psychophysical substrate of the P pathway, the chromatic system (Weingarten, 1972; Nissen & Pokorny, 1977; Bowen, 1981; Schwartz & Loop, 1982; Ueno, Pokorny & Smith, 1985; Ueno & *Department of Biomedical Sciences, Southern College of Optometry, 1245 Madison Avenue, Memphis, TN 38104, U.S.A.
Parve system
Swanson, 1989). Typically, these experiments use suprathreshold stimuli which are presented in either a hue substitution or increment mode. Schwartz and Loop (1982) studied visual latencies of the chromatic and achromatic systems by obtaining reaction time distributions (RTDs) in response to nearthreshold spatially coincident increments of long duration. A visual inspection of these RTDs suggests the achromatic system responds with a shorter latency than the chromatic system. In addition, it appears that the temporal window of response (RTD width) is less for the achromatic system. The results are comparable to those obtained by Tolhurst (1975) with low and high spatial frequency gratings. RTDs to near-threshold stimuli offer the advantage of showing the distribution of reaction times over the duration of the stimulus. This allows not only the simple reaction time to be determined, but also allows inferences with regard to the characteristics of the underlying neural response. For example, Lennie (1981) has presented a model of simple reaction time which assumes detection to occur when the cumulative neural discharge (in response to a stimulus) reaches a criterion value above the cumulative maintained discharge rate. This model, which was initially proposed by Barlow and Levick (1969), leads to the following predictions: (1) all else being equaI, a transient neural response will result in a shorter visual latency than will a sustained output; and (2) a transient neural response will result in less variability in reaction times than will a more sustained response. Lennie (198 1)provided psychophysical evidence for this model by controlling the time course of neural discharge through variation of the stimulus duration and the background illumination. Lennie did not attempt to compare transient and sustained neural channels.
2087
2088
STEVEN
H. SCHWARTZ
The above model of reaction time has implications for the shape of RTDs reflecting detection by the achromatic or chromatic system. Since threshold must be reached while the neuron is responding (Lennie, 1981), a transient neural response will produce a RTD with less width than will a sustained response. Moreover, given that all other neural characteristics (including conduction velocity) are the same, a transient response will produce a RTD which peaks earlier. Consequently, if the achromatic and chromatic systems are subserved by neural systems which show a transient/sustained dichotomy, this will be reflected in differences in their RTDs. In the current study, RTDs obtained for four naive observers and the author are evaluated. Near threshold increments, of long duration, were used to study the achromatic and chromatic systems. The peak response latency and the RTD width are compared for the two systems. The results are consistent with the notion that near-threshold increments identify systems, presumably corresponding to the M and P pathways, with different neural discharge characteristics. The Methods and Results are presented in two sections. In the first section, data are presented for conditions where detectability for the chromatic and achromatic systems is made approximately equal. Stimuli are detected on about 79% of the trials for both systems. This equality of detectability is used as a basis for comparison of the two stimulus conditions. A potential flaw of the above approach is that the criteria for detection by the chromatic and achromatic systems may be different. For example, a subject may use a less strict criterion for detection of chromatic stimuli than achromatic stimuli. Consequently, the chromatic stimuli would be closer to threshold than the achromatic stimuli: this difference in threshold criterion could contaminate the results, This concern is addressed in the second experiment. Thresholds were determined using a two-alternative forced-choice methodology: this reduces the role of the threshold criterion. RTDs were obtained for the two systems using stimuli at known positions relative to threshold.
stimulus and background were spatially coincident and 4deg in diameter. Diameter was controlled by a single aperture located prior to the final lens of the system. Head position was controlled with a dental bite. Stimulus and background intensity were controlled by Kodak neutral density wedges. The background intensity was 1000 td (Nygaard & Frumkes, 1982). One cavity interference filters were used for chromatic stimulus presentation. Peak transmission, as measured with an International Light Spectroradiometer, was within 3 nm of manufacturer specifications. On those occasions where the background was chromatic, a Jenear interference wedge was used. Stimulus presentation was accomplished with solid state programming modules interfaced with a Vincent Electronic’s shutter and a printout counter. Temporal presentation was step onset and offset with durations of 1000 msec for the chromatic system and 1000 or 1500 msec for the achromatic system. Reaction time was recorded to the nearest msec. Subjects wore headphones playing white noise to mask the shutter noise. Isolation of the chromatic and achromatic systems was accomplished with stimulus parameters suggested by the work of King-Smith and Carden (1976). Nearthreshold monochromatic increments (680, 620, 440 and 400 nm) upon a white background were used to obtain RTDs for the chromatic system. Homochromatic increments (white on white, 620 on 620 nm, or 540 on 540 nm) were used to study the achromatic system. Procedure A single experimental session consisted of 100 (subjects CG and WM) or 150 (subject BP) trials under one set of stimulus conditions. A RTD was plotted for each experimental session. The subject depressed a telegraph key. After a random pretime period, the stimulus was presented. Subjects were told to respond as soon as they saw the stimulus. Stimuli were not response terminated. Blank trials constituted approx. 10% of the trials. Experiment 2 Subjects
METHOD
Experiment
I
Subjects Two women (WM and CG) and one male (BP) in their mid-twenties participated in the study. The subjects were naive with regard to the basic design, aims and theoretical considerations of the experiment. All demonstrated normal color matching on a Nagel I anomaloscope. Stimuli and apparatus A two-channel Maxwellian view was used for stimulus presentation. A single 12-V tungsten-halogen bulb, imaged entirely within the subject’s pupil, served as the light source. An artificial pupil was not used. The
A naive observer BD (male, mid-twenties) and the author SHS (male, mid-thirties) participated in this phase of the study. Both subjects demonstrated normal color matching on a Nagel I anomaloscope. Stimulus and apparatus Stimuli were presented with a two-channel Maxwellian view system. 12-V tungsten-halogen bulbs served as light sources for each channel. Chromatic characteristics were controlled with a monochrometer located in the stimulus channel. Stimulus duration was 1000 msec. The background and stimulus were spatially coincident and 5 deg in diameter. Other features of the stimulus and apparatus are the same as those for Expt 1.
2089
REACTION TIME DISTRIBUTIONS
Procedure
I I
20
Thresholds for homochromatic (white on white) and heterochromatic (440 nm on white) stimuli were determined with a two-alternative forced-choice procedure. The stimulus was present in one of two sequentially presented temporal windows. The subject’s task was to choose the correct window. Stimulus intensity and blank arrangement were in random order. 20 or 40 trials were conducted at each intensity. A psychometric function was constructed by hand and the threshold was read directly off the plot. RTDs were obtained in a manner similar to the first phase of the experiments. Each RTD is based on 100 correct responses.
w 6201620
wm 620/620
RESULTS Experiment
J
1
Figure 1 shows representative RTDs for the homochromatic condition of white on white for all three subjects. The RTDs are bimodal with responses tightly clustered around stimulus onset and offset. This condition reflects detection by the achromatic system, and the RTDs show characteristics of those previously published (Schwartz & Loop, 1982). Figure 2 shows RTDs for homochromatic increments obtained with a monochromatic background and stimulus; these RTDs are similar to the white on white condition. Figure 3 displays a number of RTDs obtained with stimuli detected by the chromatic system. Increments were either 680, 620, 440 or 400 nm, presented upon a white background (heterochromatic increments). Three features distinguish these RTDs from those obtained with homochromatic increments. The two that concern us for now are the longer latencies and broader widths
20
L.
WIW
cg
II: wm
1000
0 r*actlon
,,m.
1500 I,.C
FIGURE 1. Representative RTDs for the homochromatic condition of a white increment on a white background. Each RTD representsa single experimental session for each subject. The bin width is 50 msec. Percentage detection was (a) 85.3%, (b) 81.0% and (c) 85.5%.
0
1
lclmuiu8
600 1000 1600 r*8cllon Ill-n. In,.5
LlGURE 2. Representative RTDs for the homochromatic condition of a spectral increment presented on a background of the same wavelength. Percentage detection was (a) 78.2%, (b) 82.2% and (c) 78.6%.
of the onset distributions. In addition, these RTDs are more polar in naturethey tend to be unimodal suggesting an asymmetry of sensitivity to stimulus onset and offset. (This is the subject of further investigation.) When comparing Figs 1 and 2 with Fig. 3, note that the overall detection rates are comparable. It would have been possible to obtain RTDs for both systems in which the detection rates for the onset modes alone were equated. Since detection by the achromatic system results in bimodal histograms, with many of the responses occurring at stimulus offset, it would have been necessary to increase the homochromatic stimulus intensity to obtain equality of onset detectability. This would have made the experimental effects more striking: the achromatic latency would have decreased and the RTD width would have narrowed. Figure 4 displays typical homochromatic and heterochromatic RTDs plotted on the same axis. The differences between the two conditions are apparent when the data are presented in this fashion. In order to apply a statistical analysis to the RTD data, it is necessary to choose parameters which reflect visual latency and RTD width. The mode of the onset distribution was chosen to reflect visual latency. The mode has the feature of being independent of the criterion selected to ascertain the RTD width. (The mean and median would be dependent on the RTD width.) In those instances where two bins were the same height, the fastest time bin was used. RTD width was determined using the following rules: (1) the beginning of the histogram was taken to be the first bin with at least two responses, (2) the histogram was assumed to end when two empty bins were encountered, and (3) the final bin with at least two responses was taken as the termination point of the histogram.
STEVEN H. SCHWARTZ
J
0
808
reaction
1000
t800
tlm*
m8ec
0
lHl0
1oQQ~
1880
FIGURE 3. Representative RTDs for the heterochromatic condition of spectral increments upon a white background. Each column represents one subject and each row represents a different chromatic stimtxl~s. Pemntage detection was (a) 73,8%, (b) 79.0%, (c) 86.1%, (d) 88.8%,(e) 82.3%, (f) 85.9%, (g) 84.3%, (h) 88.4%, (i) 78.3%, fi) 77.8%,(k) 73.9%, and (1) 85.5%.
A t-test (one tail) was used to analyze the data-and the results are given in Table 1. Differences are found in latency and RTD width for ali three subjects at a rehability of P < 1).005 (Hays, 1981). Averag&g data for all three subjects, the achromatic system shows a latency 93 msec shorter than the chromatic system. The average RTD width for the chromatic system is 240 msee greater than that for the achromatic system. Experiment 2 I
b
wm
bp
reaction
tlme
msec
FIGURE 4. Representative homochromatic (dots) and heterocbromatic RTDs for each subjwt on the same axis. Percentage detection for the homochromatic and heterochromatic conditions, respectively, was (a) 85.5 and 86.1%, (b) 81.0 and 84.3%, and (c) 85.5 and 75.9%.
This phase of the experiment was designed to address the possibility that variation in the subjects’ detection criteria could confound interpretation of the results. Thresholds were determined with a forced-choice method and stimuli are referenced to these threshold values. Summary results of RTDs obtained for both subjects are presented in Table 2. When the homochromati~ and ~e~r~romati~ stimuli are at the same value, relative to threshold, substantial differences are found in theReak latency and RTD width. This is seen in the data for sub&& BD where both categories of stimuli are at 1.6 x threshold, These differences in peak latency and RTD width are still apparent when a more intense heterochromatic stimulus is compared with a relatively. less intense homochromatic stimulus. For subject BD&eterochromatic stimuli at 2.3 x and 3.6 x threshold both produce significant results when compared with the homochromatic stimulus at I.6 x threshold. Subject SETS shows a similar result. Heterochromatic stimuli at both 1.5 x and 3.0 x threshold yield slower peak latex&s and wider RTD widths than do homochromatic stimuli at 1.3 x threshold.
REACTION
2091
TfME DISTRIBUTIONS
TABLE 1. Summary data for Expt 1
Condition
N
CG
Homochromatic Heterochromatic
4 8
71.3 78.3
337.50 462.50
5.64.
250.00 487.50
4.61*
BP
Homochromatic Heterochromatic
5 7
77.5 81.3
360.00 435.71
4.34+
370.00 650.00
3.79*
WM
Homochromatic Heterochromatic
6 8
80.0 80.8
383.33 462.50
3.75*
266.67 468.75
4.39*
Subject
Mean on peak latency (m=c)
Mean on width (m=c)
Meall percentage detection
t
t
*P < 0.005.
TABLE 2. Summary data for Expt 2
Subject BD
SHS
Condition
N
Position relative to threshold
Mean on peak latency (msec)
Mean on width
t
Homochromatic Heterochromatic
6 5
1.6x 1.6x
308 480
7.71.
275 610
5.24”
Homochromatic Heterochromatic
6 7
1.6x 2.3 x
308 407
3.19*
275 457
3.52’
Homochromatic Heterochromatic
6 6
1.6x 3.6 x
308 375
3.21.
275 366
2.23t
Homochromatic Heterochromatic
5 4
1.3x 1.5x
300 412
8.83*
230 307
4.42’
Homochromatic Heterochromatic
5 5
1.3x 3.0 x
300 350
2.82t
230 330
3.16t
(-4
t
‘P < 0.005; tP < 0.025.
RTDs reflecting detection by the chromatic system peak later and are wider than those for the achromatic system. These differences are present when the two sets of histograms are equated for overall detectability (Expt 1). They are also present when the experiment is repeated with stimuli at the same positions relative to threshold. Moreover, these differences are still present when the stimuli for the chromatic system are more intense, relative to threshold, than those for the achromatic system. DISCUSSION RTDs reflecting detection by the achromatic system show shorter latencies and more tightly clustered responses than do RTDs associated with the chromatic system. These results suggest that the neural elements underlying the achromatic system respond in a more transient fashion than those subserving the chromatic system. These conclusions follow from Lennie’s (1981) model of reaction time which was discussed in the introduction. A basic distinction between M and P pathways is the time-course of their neural discharge: neurons in the M pathway respond in a more transient fashion than those in the P pathway (DeMonasterio & Gouras, 1975; Dreher et al., 1976; Schiller & Malpeli, 1978). Moreover, the P pathway is adapted to code hue information while the M pathway is likely involved in brightness discriminations. Given these characteristics, it is plausible that the RTDs presented in this paper reflect detection by the
M and P pathways. The RTDs for the achromatic system peak at a shorter latency and are relatively narrow because of the transient nature of the M system. The RTDs for the chromatic system peak later and are wider due to the more sustained nature of the P pathway. Conduction velocity differences (approx. 15 msec) between the M and P pathways are too small to account for the differences noted in this paper (Marroco, 1976). Drawing parallels between psychophysical observations and physiology is fraught with the risk of finding an “illusory correlation”. Wasserman and Kong (1974) provide an excellent illustration of this conceptual error. Investigators (Le Grand, 1968) have attempted to link the Broca-Sulzer effect with the neural response to a long duration stimulus. Wasserman and Kong (1974) point out that although the waveform for these two phenomena is similar, the independent variables are different. For the Broca-Sulzer effect, the independent variable is stimulus duration while with neural discharge the independent variable is time. Therefore, the apparent correlation between the two is fallacious. The use of RTDs to study the underlying neural discharge does not suffer from the same conceptual flaw as does use of the Broca-Sulzer effect; RTDs show the frequency of response as a function of time. Consequently, RTDs may offer a useful psychophysical tool for investigating the relationship between sensory neural waveforms and perception.
2092
STEVEN H. SCHWARTZ REFERENCES
geniculate nucleus. Conduction velocities and response properties. Journal of Neurophysiology, 39, 340-353.
Barlow, H. B. & Levick, W. R. (1969). Three factors limiting the reliable detection of light by retinal ganglion cells of the cat. Journal of Physiology, London, 200, l-24.
Bowen, R. W. (1981). Latencies for chromatic and achromatic visual mechanisms. Vision Research, 21, 1457-1466. De Monasterio, F. M. & Gouras, P. (1975). Functional properties of ganglion cells of the rhesus monkey retina. Journal of Physiology, London, 251, 167--l 95.
Dreher, B., Fukada, Y. & Rodieck, R. W. (1976). Identification, classification and anatomical segregation of cells with X-like and Y-like properties in the lateral geniculate nucleus of old-world primates. Journal of Physiology, London, 258, 433-452. Hays, W. L. (1981). Statistics. New York: Holt (Rinehart & Winston). King-Smith, P. E. & Carden, D. (1976). Luminance and opponentcolor contributions to visual detection and adaptation and to temporal and spatial integration. Journal of the Optical Society of America, 66, 709717. Le Grand, Y. (1968). Light, color and oision. London: Chapman &
Hall. Lennie, P. (1981). The physiological basis of variations in visual latency. Vision Research, 21, 815-824. Livingstone, M. S. St Hubel, D. H. (1987). Psychophysical evidence for separate channeis for the perception of form, color, movement and depth. Journal of Neuroscience, 7, 3416-3468. Marroco, R. T. (1976). Sustained and transient cells in monkey lateral
Nissen, M. J. & Pokorny, J. (1977). Wavelength effects on simple reaction time. Perception and Psychophysics, 22, 457462. Nygaard, R. W. & Frumkes, T. E. (1982). Calibration of the retinal illuminance provided by Maxwellian views. Vision Research, 22, 433434.
Schiller, P. H. & Malpeli, J. G. (1978). Functional specificity of lateral geniculate nucleus laminae of the rhesus monkey. Journal afNeurophysiology, 41, 788-797.
Schwartz, S. H. & Loop, M. S. (1982). Evidence for transient luminance and quasi-sustained color mechanisms. Vision Research, 22, 445447.
Shapley, R. (1990). Visual sensitivity and parallel retinocortical pathways. Annual Review of Psychology, 41, 635658. Tolhurst, D. J. (1975). Reaction times in the detection of gratings by human observers: A probabilistic mechanism. Vision Research, 15, 1143-I 149.
Ueno, T. & Swanson, W. H. (1989). Response pooling between chromatic and luminance systems. Vision Research, 29, 325-333. Ueno, T., Pokorny, J. & Smith, V. C. (1985). Reaction times to chromatic stimuli. Vision Research, 25, 162331627. Wasserman, G. S. & Kong, K. L (1974). Illusory correlation of brightness enhancement and transients in the nervous system. Science, 184, 911-913.
Weingarten, F. S. (1972). Wavelength effect on visual latency. Science, 176, 692694.
VisionRes. Vol. 32, No. 11, Pp. 2087-2092,1992 Printed in Great Britain. All rights reserved
Reaction Time Distributions and Their Relationship to the Transient/Sustained Nature of the Neural Discharge STEVEN
H. SCHWARTZ*
Received 29 January 1992; in revised form 30 April 1992
Reaction time distributions (RTDs) were determined in response to near-threshold increments of long duration. Stimulus parameters were selected to isolate the chromatic and achromatic systems. The RTDs for the achromatic system peak sooner and are more narrow than those obtained for the chromatic system. These results are analyzed in terms of the neural discharge pattern of parvo and magno pathways. Color vision Reaction times Magno system
Temporal vision
Chromatic system Achromatic system
It is widely assumed that visual information in the primate is transmitted along two major retina-cortical pathways, referred to as the magno (M) and parvo (P) pathways. These pathways are anatomically distinct, up through at least visual area 2 (Livingstone & Hubel, 1987). The neurons which constitute these pathways show distinct morphologies and different neurophysiological characteristics (De Monasterio & Gouras, 1975; Dreher, Fukada & Rodieck, 1976; Schiller & Malpeli, 1978). P cells, located in the dorsal four layers of the dorsal lateral geniculate nucleus (dLGN) are smaller than M cells, which are confined to the two ventral most layers. P cells show wavelength opponency, slow conduction velocities and sustained responses to a step input. By contrast, M cells show weak, if any, color opponency, fast conduction velocities and relatively transient responses to a step input. It has been suggested that the P pathway codes hue (or color) information and the M pathway transmits luminance (or brightness) information (Shapley, 1990). Given the physiological characteristics of the M and P pathways, one can put forward the hypothesis that stimuli detected by the P pathway will be detected with a longer visual latency than those detected by the M pathway. A number of psychophysical studies have demonstrated that stimuli detected by the putative psychophysical counterpart of the M pathway, the achromatic system, are seen with a shorter visual latency than those detected by the psychophysical substrate of the P pathway, the chromatic system (Weingarten, 1972; Nissen & Pokorny, 1977; Bowen, 1981; Schwartz & Loop, 1982; Ueno, Pokorny & Smith, 1985; Ueno & *Department of Biomedical Sciences, Southern College of Optometry, 1245 Madison Avenue, Memphis, TN 38104, U.S.A.
Parve system
Swanson, 1989). Typically, these experiments use suprathreshold stimuli which are presented in either a hue substitution or increment mode. Schwartz and Loop (1982) studied visual latencies of the chromatic and achromatic systems by obtaining reaction time distributions (RTDs) in response to nearthreshold spatially coincident increments of long duration. A visual inspection of these RTDs suggests the achromatic system responds with a shorter latency than the chromatic system. In addition, it appears that the temporal window of response (RTD width) is less for the achromatic system. The results are comparable to those obtained by Tolhurst (1975) with low and high spatial frequency gratings. RTDs to near-threshold stimuli offer the advantage of showing the distribution of reaction times over the duration of the stimulus. This allows not only the simple reaction time to be determined, but also allows inferences with regard to the characteristics of the underlying neural response. For example, Lennie (1981) has presented a model of simple reaction time which assumes detection to occur when the cumulative neural discharge (in response to a stimulus) reaches a criterion value above the cumulative maintained discharge rate. This model, which was initially proposed by Barlow and Levick (1969), leads to the following predictions: (1) all else being equaI, a transient neural response will result in a shorter visual latency than will a sustained output; and (2) a transient neural response will result in less variability in reaction times than will a more sustained response. Lennie (198 1)provided psychophysical evidence for this model by controlling the time course of neural discharge through variation of the stimulus duration and the background illumination. Lennie did not attempt to compare transient and sustained neural channels.
2087
2088
STEVEN
H. SCHWARTZ
The above model of reaction time has implications for the shape of RTDs reflecting detection by the achromatic or chromatic system. Since threshold must be reached while the neuron is responding (Lennie, 1981), a transient neural response will produce a RTD with less width than will a sustained response. Moreover, given that all other neural characteristics (including conduction velocity) are the same, a transient response will produce a RTD which peaks earlier. Consequently, if the achromatic and chromatic systems are subserved by neural systems which show a transient/sustained dichotomy, this will be reflected in differences in their RTDs. In the current study, RTDs obtained for four naive observers and the author are evaluated. Near threshold increments, of long duration, were used to study the achromatic and chromatic systems. The peak response latency and the RTD width are compared for the two systems. The results are consistent with the notion that near-threshold increments identify systems, presumably corresponding to the M and P pathways, with different neural discharge characteristics. The Methods and Results are presented in two sections. In the first section, data are presented for conditions where detectability for the chromatic and achromatic systems is made approximately equal. Stimuli are detected on about 79% of the trials for both systems. This equality of detectability is used as a basis for comparison of the two stimulus conditions. A potential flaw of the above approach is that the criteria for detection by the chromatic and achromatic systems may be different. For example, a subject may use a less strict criterion for detection of chromatic stimuli than achromatic stimuli. Consequently, the chromatic stimuli would be closer to threshold than the achromatic stimuli: this difference in threshold criterion could contaminate the results, This concern is addressed in the second experiment. Thresholds were determined using a two-alternative forced-choice methodology: this reduces the role of the threshold criterion. RTDs were obtained for the two systems using stimuli at known positions relative to threshold.
stimulus and background were spatially coincident and 4deg in diameter. Diameter was controlled by a single aperture located prior to the final lens of the system. Head position was controlled with a dental bite. Stimulus and background intensity were controlled by Kodak neutral density wedges. The background intensity was 1000 td (Nygaard & Frumkes, 1982). One cavity interference filters were used for chromatic stimulus presentation. Peak transmission, as measured with an International Light Spectroradiometer, was within 3 nm of manufacturer specifications. On those occasions where the background was chromatic, a Jenear interference wedge was used. Stimulus presentation was accomplished with solid state programming modules interfaced with a Vincent Electronic’s shutter and a printout counter. Temporal presentation was step onset and offset with durations of 1000 msec for the chromatic system and 1000 or 1500 msec for the achromatic system. Reaction time was recorded to the nearest msec. Subjects wore headphones playing white noise to mask the shutter noise. Isolation of the chromatic and achromatic systems was accomplished with stimulus parameters suggested by the work of King-Smith and Carden (1976). Nearthreshold monochromatic increments (680, 620, 440 and 400 nm) upon a white background were used to obtain RTDs for the chromatic system. Homochromatic increments (white on white, 620 on 620 nm, or 540 on 540 nm) were used to study the achromatic system. Procedure A single experimental session consisted of 100 (subjects CG and WM) or 150 (subject BP) trials under one set of stimulus conditions. A RTD was plotted for each experimental session. The subject depressed a telegraph key. After a random pretime period, the stimulus was presented. Subjects were told to respond as soon as they saw the stimulus. Stimuli were not response terminated. Blank trials constituted approx. 10% of the trials. Experiment 2 Subjects
METHOD
Experiment
I
Subjects Two women (WM and CG) and one male (BP) in their mid-twenties participated in the study. The subjects were naive with regard to the basic design, aims and theoretical considerations of the experiment. All demonstrated normal color matching on a Nagel I anomaloscope. Stimuli and apparatus A two-channel Maxwellian view was used for stimulus presentation. A single 12-V tungsten-halogen bulb, imaged entirely within the subject’s pupil, served as the light source. An artificial pupil was not used. The
A naive observer BD (male, mid-twenties) and the author SHS (male, mid-thirties) participated in this phase of the study. Both subjects demonstrated normal color matching on a Nagel I anomaloscope. Stimulus and apparatus Stimuli were presented with a two-channel Maxwellian view system. 12-V tungsten-halogen bulbs served as light sources for each channel. Chromatic characteristics were controlled with a monochrometer located in the stimulus channel. Stimulus duration was 1000 msec. The background and stimulus were spatially coincident and 5 deg in diameter. Other features of the stimulus and apparatus are the same as those for Expt 1.
2089
REACTION TIME DISTRIBUTIONS
Procedure
I I
20
Thresholds for homochromatic (white on white) and heterochromatic (440 nm on white) stimuli were determined with a two-alternative forced-choice procedure. The stimulus was present in one of two sequentially presented temporal windows. The subject’s task was to choose the correct window. Stimulus intensity and blank arrangement were in random order. 20 or 40 trials were conducted at each intensity. A psychometric function was constructed by hand and the threshold was read directly off the plot. RTDs were obtained in a manner similar to the first phase of the experiments. Each RTD is based on 100 correct responses.
w 6201620
wm 620/620
RESULTS Experiment
J
1
Figure 1 shows representative RTDs for the homochromatic condition of white on white for all three subjects. The RTDs are bimodal with responses tightly clustered around stimulus onset and offset. This condition reflects detection by the achromatic system, and the RTDs show characteristics of those previously published (Schwartz & Loop, 1982). Figure 2 shows RTDs for homochromatic increments obtained with a monochromatic background and stimulus; these RTDs are similar to the white on white condition. Figure 3 displays a number of RTDs obtained with stimuli detected by the chromatic system. Increments were either 680, 620, 440 or 400 nm, presented upon a white background (heterochromatic increments). Three features distinguish these RTDs from those obtained with homochromatic increments. The two that concern us for now are the longer latencies and broader widths
20
L.
WIW
cg
II: wm
1000
0 r*actlon
,,m.
1500 I,.C
FIGURE 1. Representative RTDs for the homochromatic condition of a white increment on a white background. Each RTD representsa single experimental session for each subject. The bin width is 50 msec. Percentage detection was (a) 85.3%, (b) 81.0% and (c) 85.5%.
0
1
lclmuiu8
600 1000 1600 r*8cllon Ill-n. In,.5
LlGURE 2. Representative RTDs for the homochromatic condition of a spectral increment presented on a background of the same wavelength. Percentage detection was (a) 78.2%, (b) 82.2% and (c) 78.6%.
of the onset distributions. In addition, these RTDs are more polar in naturethey tend to be unimodal suggesting an asymmetry of sensitivity to stimulus onset and offset. (This is the subject of further investigation.) When comparing Figs 1 and 2 with Fig. 3, note that the overall detection rates are comparable. It would have been possible to obtain RTDs for both systems in which the detection rates for the onset modes alone were equated. Since detection by the achromatic system results in bimodal histograms, with many of the responses occurring at stimulus offset, it would have been necessary to increase the homochromatic stimulus intensity to obtain equality of onset detectability. This would have made the experimental effects more striking: the achromatic latency would have decreased and the RTD width would have narrowed. Figure 4 displays typical homochromatic and heterochromatic RTDs plotted on the same axis. The differences between the two conditions are apparent when the data are presented in this fashion. In order to apply a statistical analysis to the RTD data, it is necessary to choose parameters which reflect visual latency and RTD width. The mode of the onset distribution was chosen to reflect visual latency. The mode has the feature of being independent of the criterion selected to ascertain the RTD width. (The mean and median would be dependent on the RTD width.) In those instances where two bins were the same height, the fastest time bin was used. RTD width was determined using the following rules: (1) the beginning of the histogram was taken to be the first bin with at least two responses, (2) the histogram was assumed to end when two empty bins were encountered, and (3) the final bin with at least two responses was taken as the termination point of the histogram.
STEVEN H. SCHWARTZ
J
0
808
reaction
1000
t800
tlm*
m8ec
0
lHl0
1oQQ~
1880
FIGURE 3. Representative RTDs for the heterochromatic condition of spectral increments upon a white background. Each column represents one subject and each row represents a different chromatic stimtxl~s. Pemntage detection was (a) 73,8%, (b) 79.0%, (c) 86.1%, (d) 88.8%,(e) 82.3%, (f) 85.9%, (g) 84.3%, (h) 88.4%, (i) 78.3%, fi) 77.8%,(k) 73.9%, and (1) 85.5%.
A t-test (one tail) was used to analyze the data-and the results are given in Table 1. Differences are found in latency and RTD width for ali three subjects at a rehability of P < 1).005 (Hays, 1981). Averag&g data for all three subjects, the achromatic system shows a latency 93 msec shorter than the chromatic system. The average RTD width for the chromatic system is 240 msee greater than that for the achromatic system. Experiment 2 I
b
wm
bp
reaction
tlme
msec
FIGURE 4. Representative homochromatic (dots) and heterocbromatic RTDs for each subjwt on the same axis. Percentage detection for the homochromatic and heterochromatic conditions, respectively, was (a) 85.5 and 86.1%, (b) 81.0 and 84.3%, and (c) 85.5 and 75.9%.
This phase of the experiment was designed to address the possibility that variation in the subjects’ detection criteria could confound interpretation of the results. Thresholds were determined with a forced-choice method and stimuli are referenced to these threshold values. Summary results of RTDs obtained for both subjects are presented in Table 2. When the homochromati~ and ~e~r~romati~ stimuli are at the same value, relative to threshold, substantial differences are found in theReak latency and RTD width. This is seen in the data for sub&& BD where both categories of stimuli are at 1.6 x threshold, These differences in peak latency and RTD width are still apparent when a more intense heterochromatic stimulus is compared with a relatively. less intense homochromatic stimulus. For subject BD&eterochromatic stimuli at 2.3 x and 3.6 x threshold both produce significant results when compared with the homochromatic stimulus at I.6 x threshold. Subject SETS shows a similar result. Heterochromatic stimuli at both 1.5 x and 3.0 x threshold yield slower peak latex&s and wider RTD widths than do homochromatic stimuli at 1.3 x threshold.
REACTION
2091
TfME DISTRIBUTIONS
TABLE 1. Summary data for Expt 1
Condition
N
CG
Homochromatic Heterochromatic
4 8
71.3 78.3
337.50 462.50
5.64.
250.00 487.50
4.61*
BP
Homochromatic Heterochromatic
5 7
77.5 81.3
360.00 435.71
4.34+
370.00 650.00
3.79*
WM
Homochromatic Heterochromatic
6 8
80.0 80.8
383.33 462.50
3.75*
266.67 468.75
4.39*
Subject
Mean on peak latency (m=c)
Mean on width (m=c)
Meall percentage detection
t
t
*P < 0.005.
TABLE 2. Summary data for Expt 2
Subject BD
SHS
Condition
N
Position relative to threshold
Mean on peak latency (msec)
Mean on width
t
Homochromatic Heterochromatic
6 5
1.6x 1.6x
308 480
7.71.
275 610
5.24”
Homochromatic Heterochromatic
6 7
1.6x 2.3 x
308 407
3.19*
275 457
3.52’
Homochromatic Heterochromatic
6 6
1.6x 3.6 x
308 375
3.21.
275 366
2.23t
Homochromatic Heterochromatic
5 4
1.3x 1.5x
300 412
8.83*
230 307
4.42’
Homochromatic Heterochromatic
5 5
1.3x 3.0 x
300 350
2.82t
230 330
3.16t
(-4
t
‘P < 0.005; tP < 0.025.
RTDs reflecting detection by the chromatic system peak later and are wider than those for the achromatic system. These differences are present when the two sets of histograms are equated for overall detectability (Expt 1). They are also present when the experiment is repeated with stimuli at the same positions relative to threshold. Moreover, these differences are still present when the stimuli for the chromatic system are more intense, relative to threshold, than those for the achromatic system. DISCUSSION RTDs reflecting detection by the achromatic system show shorter latencies and more tightly clustered responses than do RTDs associated with the chromatic system. These results suggest that the neural elements underlying the achromatic system respond in a more transient fashion than those subserving the chromatic system. These conclusions follow from Lennie’s (1981) model of reaction time which was discussed in the introduction. A basic distinction between M and P pathways is the time-course of their neural discharge: neurons in the M pathway respond in a more transient fashion than those in the P pathway (DeMonasterio & Gouras, 1975; Dreher et al., 1976; Schiller & Malpeli, 1978). Moreover, the P pathway is adapted to code hue information while the M pathway is likely involved in brightness discriminations. Given these characteristics, it is plausible that the RTDs presented in this paper reflect detection by the
M and P pathways. The RTDs for the achromatic system peak at a shorter latency and are relatively narrow because of the transient nature of the M system. The RTDs for the chromatic system peak later and are wider due to the more sustained nature of the P pathway. Conduction velocity differences (approx. 15 msec) between the M and P pathways are too small to account for the differences noted in this paper (Marroco, 1976). Drawing parallels between psychophysical observations and physiology is fraught with the risk of finding an “illusory correlation”. Wasserman and Kong (1974) provide an excellent illustration of this conceptual error. Investigators (Le Grand, 1968) have attempted to link the Broca-Sulzer effect with the neural response to a long duration stimulus. Wasserman and Kong (1974) point out that although the waveform for these two phenomena is similar, the independent variables are different. For the Broca-Sulzer effect, the independent variable is stimulus duration while with neural discharge the independent variable is time. Therefore, the apparent correlation between the two is fallacious. The use of RTDs to study the underlying neural discharge does not suffer from the same conceptual flaw as does use of the Broca-Sulzer effect; RTDs show the frequency of response as a function of time. Consequently, RTDs may offer a useful psychophysical tool for investigating the relationship between sensory neural waveforms and perception.
2092
STEVEN H. SCHWARTZ REFERENCES
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