Luminance Discrimination Task Research Articles

An initial 'snapshot' of sensory information biases the likelihood and speed of subsequent changes of mind.

We often need to rapidly change our mind about perceptual decisions in order to account for new information and correct mistakes. One fundamental, unresolved question is whether information processed prior to a decision being made ('pre-decisional information') has any influence on the likelihood and speed with which that decision is reversed. We investigated this using a luminance discrimination task in which participants indicated which of two flickering greyscale squares was brightest. Following an initial decision, the stimuli briefly remained on screen, and participants could change their response. Using psychophysical reverse correlation, we examined how moment-to-moment fluctuations in stimulus luminance affected participants' decisions. This revealed that the strength of even the very earliest (pre-decisional) evidence was associated with the likelihood and speed of later changes of mind. To account for this effect, we propose an extended diffusion model in which an initial 'snapshot' of sensory information biases ongoing evidence accumulation.

Chronically implantable LED arrays for behavioral optogenetics in primates.

Optogenetic methods have been widely used in rodent brains, but remain relatively under-developed for nonhuman primates such as rhesus macaques, an animal model with a large brain expressing sophisticated sensory, motor and cognitive behaviors. To address challenges in behavioral optogenetics in large brains, we developed Opto-Array, a chronically implantable array of light-emitting diodes for high-throughput optogenetic perturbation. We demonstrated that optogenetic silencing in the macaque primary visual cortex with the help of the Opto-Array results in reliable retinotopic visual deficits in a luminance discrimination task. We separately confirmed that Opto-Array illumination results in local neural silencing, and that behavioral effects are not due to tissue heating. These results demonstrate the effectiveness of the Opto-Array for behavioral optogenetic applications in large brains.

Perceptual decision confidence is sensitive to forgone physical effort expenditure

Contemporary theoretical accounts of metacognition propose that action-related information is used in the computation of perceptual decision confidence. We investigated whether the amount of expended physical effort, or the ‘motoric sunk cost’ of a decision, influences perceptual decision confidence judgements in humans. In particular, we examined whether people feel more confident in decisions which required more effort to report. Forty-two participants performed a luminance discrimination task that involved identifying which of two flickering grayscale squares was brightest. Participants reported their choice by squeezing hand-held dynamometers. Across trials, the effort required to report a decision was varied across three levels (low, medium, high). Critically, participants were only aware of the required effort level on each trial once they had initiated their motor response, meaning that the varying effort requirements could not influence their initial decisions. Following each decision, participants rated their confidence in their choice. We found that participants were more confident in decisions that required greater effort to report. This suggests that humans are sensitive to motoric sunk costs and supports contemporary models of metacognition in which actions inform the computation of decision confidence.

Tracking the completion of parts into whole objects: Retinotopic activation in response to illusory figures in the lateral occipital complex

Illusory figures demonstrate the visual system’s ability to integrate separate parts into coherent, whole objects. The present study was performed to track the neuronal object construction process in human observers, by incrementally manipulating the grouping strength within a given configuration until the emergence of a whole-object representation. Two tasks were employed: First, in the spatial localization task, object completion could facilitate performance and was task-relevant, whereas it was irrelevant in the second, luminance discrimination task. Concurrent functional magnetic resonance imaging (fMRI) used spatial localizers to locate brain regions representing task-critical illusory-figure parts to investigate whether the step-wise object construction process would modulate neural activity in these localized brain regions. The results revealed that both V1 and the lateral occipital complex (LOC, with sub-regions LO1 and LO2) were involved in Kanizsa figure processing. However, completion-specific activations were found predominantly in LOC, where neural activity exhibited a modulation in accord with the configuration’s grouping strength, whether or not the configuration was relevant to performing the task at hand. Moreover, right LOC activations were confined to LO2 and responded primarily to surface and shape completions, whereas left LOC exhibited activations in both LO1 and LO2 and was related to encoding shape structures with more detail. Together, these results demonstrate that various grouping properties within a visual scene are integrated automatically in LOC, with sub-regions located in different hemispheres specializing in the component sub-processes that render completed objects.

Influence of Spatial and Chromatic Noise on Luminance Discrimination

Pseudoisochromatic figures are designed to base discrimination of a chromatic target from a background solely on the chromatic differences. This is accomplished by the introduction of luminance and spatial noise thereby eliminating these two dimensions as cues. The inverse rationale could also be applied to luminance discrimination, if spatial and chromatic noise are used to mask those cues. In this current study estimate of luminance contrast thresholds were conducted using a novel stimulus, based on the use of chromatic and spatial noise to mask the use of these cues in a luminance discrimination task. This was accomplished by presenting stimuli composed of a mosaic of circles colored randomly. A Landolt-C target differed from the background only by the luminance. The luminance contrast thresholds were estimated for different chromatic noise saturation conditions and compared to luminance contrast thresholds estimated using the same target in a non-mosaic stimulus. Moreover, the influence of the chromatic content in the noise on the luminance contrast threshold was also investigated. Luminance contrast threshold was dependent on the chromaticity noise strength. It was 10-fold higher than thresholds estimated from non-mosaic stimulus, but they were independent of colour space location in which the noise was modulated. The present study introduces a new method to investigate luminance vision intended for both basic science and clinical applications.

Prestimulus EEG Power Predicts Conscious Awareness But Not Objective Visual Performance.

Prestimulus oscillatory neural activity has been linked to perceptual outcomes during performance of psychophysical detection and discrimination tasks. Specifically, the power and phase of low frequency oscillations have been found to predict whether an upcoming weak visual target will be detected or not. However, the mechanisms by which baseline oscillatory activity influences perception remain unclear. Recent studies suggest that the frequently reported negative relationship between α power and stimulus detection may be explained by changes in detection criterion (i.e., increased target present responses regardless of whether the target was present/absent) driven by the state of neural excitability, rather than changes in visual sensitivity (i.e., more veridical percepts). Here, we recorded EEG while human participants performed a luminance discrimination task on perithreshold stimuli in combination with single-trial ratings of perceptual awareness. Our aim was to investigate whether the power and/or phase of prestimulus oscillatory activity predict discrimination accuracy and/or perceptual awareness on a trial-by-trial basis. Prestimulus power (3–28 Hz) was inversely related to perceptual awareness ratings (i.e., higher ratings in states of low prestimulus power/high excitability) but did not predict discrimination accuracy. In contrast, prestimulus oscillatory phase did not predict awareness ratings or accuracy in any frequency band. These results provide evidence that prestimulus α power influences the level of subjective awareness of threshold visual stimuli but does not influence visual sensitivity when a decision has to be made regarding stimulus features. Hence, we find a clear dissociation between the influence of ongoing neural activity on conscious awareness and objective performance.

Motion direction discrimination training reduces perceived motion repulsion.

Participants often exaggerate the perceived angular separation between two simultaneously presented motion stimuli, which is referred to as motion repulsion. The overestimation helps participants differentiate between the two superimposed motion directions, yet it causes the impairment of direction perception. Since direction perception can be refined through perceptual training, we here attempted to investigate whether the training of a direction discrimination task changes the amount of motion repulsion. Our results showed a direction-specific learning effect, which was accompanied by a reduced amount of motion repulsion both for the trained and the untrained directions. The reduction of the motion repulsion disappeared when the participants were trained on a luminance discrimination task (control experiment 1) or a speed discrimination task (control experiment 2), ruling out any possible interpretation in terms of adaptation or training-induced attentional bias. Furthermore, training with a direction discrimination task along a direction 150° away from both directions in the transparent stimulus (control experiment 3) also had little effect on the amount of motion repulsion, ruling out the contribution of task learning. The changed motion repulsion observed in the main experiment was consistent with the prediction of the recurrent model of perceptual learning. Therefore, our findings demonstrate that training in direction discrimination can benefit the precise direction perception of the transparent stimulus and provide new evidence for the recurrent model of perceptual learning.

Am I seeing myself, my friend or a stranger? The role of personal familiarity in visual distinction of body identities in the human brain

Several brain regions appear to play a role in representing different body identities. The specific contribution of each of these regions is still unclear, however. Here we investigated which brain areas enable the visual distinction between self and other bodies of different familiarity, and between familiar and unfamiliar other individuals, and moreover, where identity-specific information on the three individuals was encoded. Participants were confronted with standardized headless human body stimuli either showing the participant's own, a personally familiar or an unfamiliar other body, while performing a luminance discrimination task. Employing multivariate pattern analysis, we were able to identify areas that allowed for the distinction of self from personal familiar other bodies within the medial prefrontal cortex (mPFC) and posterior cingulate cortex/precuneus. Successful distinction of self from unfamiliar others was possible in the left middle frontal gyrus, the right inferior frontal gyrus, the left pre-supplementary motor area and the right putamen. Personally familiar others could be distinguished from unfamiliar others in the right temporoparietal junction (TPJ). An analysis of identity-specific information revealed a spatial gradient ranging from inferior posterior to superior anterior portions of the mPFC that was associated with encoding identity-related information for self via familiar to unfamiliar other bodies, respectively. Furthermore, several midline and frontal regions encoded information on more than one identity. The TPJ's role in deviance detection was underlined, as only identity-specific information on unfamiliar others was encoded here. Together, our findings suggest substantial spatial overlap in neural correlates of self and other body representation and thus, support the hypothesis of a socially-related representation of the self.

Feature-based attention is independent of object appearance

How attention interacts with low-level visual representations to give rise to perception remains a central yet controversial question in neuroscience. While several previous studies suggest that the units of attentional selection are individual objects, other evidence points instead toward lower-level features, such as an attended color or direction of motion. We used both human fMRI and psychophysics to investigate the relationship between object-based and feature-based attention. Specifically, we focused on whether feature-based attention is modulated by object appearance, comparing three conditions: (a) features appearing as one object; (b) features appearing as two separate but identical objects; (c) features appearing as two different objects. Stimuli were two random-dot fields presented bilaterally to central fixation, and object appearance was induced by the presence of one or two boxes around the fields. In the fMRI experiment, participants performed a luminance discrimination task on one side, and ignored the other side, where we probed for enhanced activity when either it was perceived as belonging to a same object, or shared features with the task side. In the psychophysical experiments, participants performed luminance discrimination on both sides with overlapping red and green dots, now attending to either the same features (red/red or green/green) or different features (red/green or green/red) on both sides. Results show that feature-based attentional enhancement exists in all three conditions, i.e., regardless whether features appear as one object, two identical objects, or two different objects. Our findings indicate that feature-based attention differs from object-based attention in that it is not dependent upon object appearance. Thus feature-based attention may be mediated by earlier cortical processes independent of perceiving visual features into well-formed objects.

Speed-accuracy tradeoffs for pursuit and saccades in a luminance discrimination task

Speed-accuracy tradeoffs for pursuit and saccades in a luminance discrimination task

Cognitively directed spatial selection in the frontal eye field in anticipation of visual stimuli to be discriminated

Single neuron activity was recorded in the frontal eye field (FEF) of monkeys trained to perform a difficult luminance discrimination task. The appearance of a cue stimulus informed the monkeys of the locations of two gray luminance stimuli that would appear within 500–1500ms. The monkeys were rewarded for making a saccade to the brighter of the two luminance stimuli, or if they were the same luminance, for making a saccade to the cue stimulus. Sixty percent (51/85) of FEF neurons exhibited elevated activity when the cue informed the monkeys that one of the luminance stimuli would appear in their response field (RF). This spatially selective anticipatory activity occurred without any visual stimulus appearing in their RF and was not related to saccade choice or latency. The responses of 27 of the anticipatory neurons (32% of the total sample) were also incompatible with the hypothesis that the activity represents saccade probability because they did not exhibit elevated activity for the cue stimulus which was the most probable saccade target. Behaviorally, monkeys exhibited improved perception at locations informed by cue than at unpredictable locations. These results provide physiological evidence that FEF serves an important role in endogenous spatial attention in addition to its well-known role in saccade production.

Shared decision signal explains performance and timing of pursuit and saccadic eye movements

Each voluntary eye movement provides physical evidence of a visuomotor choice about where and when to look. Primates choose visual targets with two types of voluntary eye movements, pursuit and saccades, although the exact mechanism underlying their coordination remains unknown. Are pursuit and saccades guided by the same decision signal? The present study compares pursuit and saccadic choices using techniques borrowed from psychophysics and models of response time. Human observers performed a luminance discrimination task and indicated their choices with eye movements. Because the stimuli moved horizontally and were offset vertically, subjects' tracking responses consisted of combinations of both pursuit and saccadic eye movements. For each of two signal strengths, we constructed speed-accuracy curves for pursuit and saccades. We found that speed-accuracy curves for pursuit and saccades have the same shape, but are time-shifted with respect to one another. We argue that this pattern occurs because pursuit and saccades share a decision signal, but utilize different response thresholds and are subject to different motor processing delays.

Hemispheric asymmetries for simple visual judgments in the split brain

While it is commonly noted that the right cerebral hemisphere is specialized for visuospatial processing, the scope and nature of this specialization remain somewhat ill defined. Our previous research with callosotomy (‘split-brain’) patients has suggested that the asymmetry may be limited to conditions that have an explicit spatial component. To investigate this we compared the performance of the divided hemispheres of two callosotomy patients on four simple visual-matching tasks. These tasks were orientation discrimination, vernier offset discrimination, size discrimination, and luminance discrimination. In each task, two stimuli were presented briefly to one visual hemifield and the patient was asked to discriminate whether they were the same or different. The first three tasks (orientation, vernier, and size) were all spatial in nature and were performed better by the right hemisphere. The luminance discrimination task, which is non-spatial, was performed equivalently by the two hemispheres. These results support the view that the fundamental difference in visual function between the hemispheres is in the ability to perform spatial discriminations.

Stimulus probability directs spatial attention: an enhancement of sensitivity in humans and monkeys

We examined whether improvements in sensory processing, defined as changes in sensitivity, could be elicited in a simple luminance discrimination task without eliciting concomitant changes in decision processing. To this end we developed a task, for use in both humans and monkeys, in which prior knowledge about where a discriminative stimulus was likely to appear (1) offered no decisional advantage in solving our task and (2) could be parametrically varied to yield a psychometric function. We found that if we parametrically varied the quality of prior knowledge, by increasing the probability, and thus the certainty, that a discriminative stimulus would appear at a particular location under these conditions, luminance discrimination improved for both human and monkey subjects. This improvement was correlated with an enhancement in sensory processing, but not with any systematic changes in decisional processing, as assessed by signal detection theory. These results suggest that (1) sensory processing and decision processing can be separated by task design and (2) systematic changes in prior knowledge about where a stimulus may appear can lead to systematic changes in sensitivity; providing a psychometric function for the influence of prior knowledge on perceptual sensitivity. Importantly, these results were obtained from both human and monkey subjects. Similar task designs could be used in physiological studies attempting to generate linking hypotheses between psychometric and neurometric functions, ultimately allowing changes in perceptual sensitivity to be linked to changes in an underlying neural substrate.

Human colour discrimination based on a non-parvocellular pathway

Background: Traditionally, colour information is assumed to be carried by neural channels in the parvocellular pathway and to be encoded in an opponent manner, while other, non-parvocellular, spectrally non-opponent channels are thought to play no part in colour vision. But is the parvocellular pathway the only way that colours can be discriminated in human vision? We studied two patients with cerebral achromatopsia, who lack conscious colour perception but are nevertheless able to make use of colour information. In particular, we investigated whether, in these patients, colour discrimination is mediated by the parvocellular pathway.Results The achromatopsic patients carried out a forced-choice colour- and luminance-discrimination task, and showed clear evidence of unconscious colour processing, consistent with previous studies. We added different types of luminance noise to see when this unconscious colour information could be masked. The results of the colour-discrimination-with-noise and the brightness-non-additivity experiments showed a double-dissociation between patients. This indicates that, in one patient, unconscious colour discrimination may be subserved by a spectrally non-opponent mechanism, which does not have the characteristics of the parvocellular pathway and which is responsive to fast flicker. Spectral sensitivity, contrast sensitivity and motion perception experiments confirmed that this patient lacks a working opponent parvocellular system. The second achromatopsic patient showed evidence of a residual parvocellular system.Conclusion Our results show that chromatic discrimination need not be mediated by neural mechanisms, the parvocellular system in particular, normally assumed to subserve conscious colour perception. Such discrimination may be mediated by a neural subsystem which responds to fast flicker, is spectrally non-opponent, and supports normal motion perception.

Tonic-phasic-channel dichotomy and Crozier's law.

The constancy of the dynamic range in a luminance-discrimination task is known as Crozier's law; an old rule says that about half of a log unit spans the range from a low to a high frequency of seeing. For our conditions the slope of the psychometric function is steeper for short than for long test flashes; Crozier's law requires a different constant when temporal parameters change. This result is substantiated by an analysis of Massof's data [Vision Res. 21,995 (1981)] on the variation of the slope of psychometric curves for different wavelengths. The change in Crozier's constant between conditions may reflect the presence of more than one detection channel. If short test flashes are detected by phasic channels and long test flashes by both phasic and tonic channels, then our result implies a shorter dynamic range for phasic (Y-cell) than for tonic (X-cell) channels.