Psychophysical Evidence Research Articles

Psychophysical evidence for interactions between visual form and motion signals during motion integration in cortical area MT

Psychophysical evidence for interactions between visual form and motion signals during motion integration in cortical area MT

Psychophysical evidence for dissociate neural assemblies encoding three dimensional convex/concave structures

Psychophysical evidence for dissociate neural assemblies encoding three dimensional convex/concave structures

Inconsistency between peri-saccadic mislocalization and compression: Evidence for separate "what" and "where" visual systems

The view of two separate "what" and "where" visual systems is supported by compelling neurophysiological evidence. However, very little direct psychophysical evidence has been presented to suggest that the two functions can be separated in neurologically intact persons. Using a peri-saccadic perception paradigm in which bars of different lengths were flashed around saccade onset, we directly measured the perceived object size (a "what" attribute) and location (a "where" attribute). We found that the perceived object location shifted toward the saccade target to show strongly compressed localization, whereas the perceived object size was not compressed accordingly. This dissociation indicates that the perceived size is not determined by spatial localization of the object boundary, providing direct psychophysical evidence to support that "what" and "where" attributes of objects are indeed processed separately.

Psychophysical evidence for internal delays in binaural detection.

The availability of an internal delay line has major consequences for performance in binaural detection tasks. It is well known that binaural thresholds vary with the interaural correlation of a noise masker, and the detrimental effect of noise decorrelation is well described by many models. When a delay line is present, however, the exact nature of the decorrelation of the noise becomes crucial. Decorrelation originating from an external delay can be compensated by internal delays, but other sources of decorrelation (e.g., mixing) cannot be undone. The special status of decorrelation from interaural delay is supported by psychoacoustic evidence. The quantitative analysis of such data, which requires a careful consideration of peripheral frequency selectivity, indicates the existence of a delay line in human listeners that can compensate accurately for external delays of up to ∼750 μm. Larger internal delays, i.e., up to 2–3 ms, are probably also operative, but performance using such large internal delays is degraded as if their application introduces noise in the internal stimulus representation. I will review the psychoacoustic evidence for internal delays, discuss the assumptions underlying their interpretation, and rebut a recent attempt to dismiss this evidence.

Structural processing in biological motion perception

To investigate the basis for biological motion perception, structural and motion information were manipulated independently in a dynamic display using a novel stimulus with multiple apertures. Performance was compared in discrimination of global motion (translation and rotation) and biological motion. When structural information in the display was eliminated but motion information was intact, human observers were able to perceive global motion yet were at chance in discriminating walking direction of biological movement. In contrast, when the display provided even noisy and impoverished structural information, walking direction became identifiable. The present findings thus provide direct psychophysical evidence that motion information is insufficient and structural information is necessary for the identification of walking direction in biological movement. These findings imply that computational models must utilize a structural representation of the human body to account for perception of biological movements.

A metanalysis of the effect of the Müller-Lyer illusion on saccadic eye movements: No general support for a dissociation of perception and oculomotor action

Milner and Goodale’s (1995) proposal of a functional division of labor between vision-for-perception and vision-for-action is supported by neuropsychological, brain-imaging, and psychophysical evidence. However, there remains considerable debate as to whether, as their proposal would predict, the effect of contextual illusions on vision-for-action can be dissociated from that on vision-for-perception. Meta-analytical efforts examining the effect of the Müller-Lyer (ML) illusion on pointing ( Bruno, Bernardis, & Gentilucci, 2008) or grasping ( Bruno & Franz, 2009) have been conducted to resolve the controversy. To complement this work, here we re-analyzed 17 papers detailing 21 independent studies investigating primary saccades to target locations that were perceptually biased by the ML illusion. Using a corrected percent illusion effect measure to compare across different studies and across experimental conditions within studies, we find that saccadic eye movements are always strongly biased by the illusion although the size of this effect can be reduced by factors such as display duration and between-trials variability in display length and orientation, possibly due to a process of saccadic adaptation. In contrast to some reports, we find no general support for differences between voluntary and reflexive saccades or between saccades performed in conjunction with a pointing movement and saccades performed without pointing. We conclude that studies on the effect of the Müller-Lyer illusion do not provide evidence for a functional dissociation between primary saccades and perception.

Psychophysical evidence for object segregation through endogenous asynchrony

Psychophysical evidence for object segregation through endogenous asynchrony

The Influence of Perceptual Training on Working Memory in Older Adults

Normal aging is associated with a degradation of perceptual abilities and a decline in higher-level cognitive functions, notably working memory. To remediate age-related deficits, cognitive training programs are increasingly being developed. However, it is not yet definitively established if, and by what mechanisms, training ameliorates effects of cognitive aging. Furthermore, a major factor impeding the success of training programs is a frequent failure of training to transfer benefits to untrained abilities. Here, we offer the first evidence of direct transfer-of-benefits from perceptual discrimination training to working memory performance in older adults. Moreover, using electroencephalography to evaluate participants before and after training, we reveal neural evidence of functional plasticity in older adult brains, such that training-induced modifications in early visual processing during stimulus encoding predict working memory accuracy improvements. These findings demonstrate the strength of the perceptual discrimination training approach by offering clear psychophysical evidence of transfer-of-benefit and a neural mechanism underlying cognitive improvement.

Asymmetry of visual sensory mechanisms: Electrophysiological, structural, and psychophysical evidences

Psychophysical visual field asymmetries are widely documented and have been attributed to anatomical anisotropies both at the retinal and cortical levels. This debate on whether such differences originate within the retina itself or are due to higher visual processing may be illuminated if concomitant anatomical, physiological, and psychophysical measures are taken in the same individuals. In the current study, we have focused on the study of objective functional and structural asymmetries at the retinal level and examined their putative correlation with visual performance asymmetries. Forty healthy participants (80 eyes; 13 male and 27 female subjects) were included in this study. Objective functional/structural asymmetries were probed using the multifocal electroretinogram (mfERG) technique and optical coherence tomography (OCT), respectively. A nasal/temporal pattern of asymmetry (nasal visual hemifield disadvantage) was found for all methods (retinal thickness, contrast sensitivity, and mfERG P1 amplitude). Furthermore, superior/inferior asymmetries could be documented only with psychophysics and structural measures. These patterns likely arise at different levels of the retina as inferred by partly independent correlation patterns. We conclude that patterns of structural/functional asymmetries arise at different levels of visual processing with a strong retinal contribution.

Discrimination contours for the perception of head-centered velocity

There is little direct psychophysical evidence that the visual system contains mechanisms tuned to head-centered velocity when observers make a smooth pursuit eye movement. Much of the evidence is implicit, relying on measurements of bias (e.g., matching and nulling). We therefore measured discrimination contours in a space dimensioned by pursuit target motion and relative motion between target and background. Within this space, lines of constant head-centered motion are parallel to the main negative diagonal, so judgments dominated by mechanisms that combine individual components should produce contours with a similar orientation. Conversely, contours oriented parallel to the cardinal axes of the space indicate judgments based on individual components. The results provided evidence for mechanisms tuned to head-centered velocity-discrimination ellipses were significantly oriented away from the cardinal axes, toward the main negative diagonal. However, ellipse orientation was considerably less steep than predicted by a pure combination of components. This suggests that observers used a mixture of two strategies across trials, one based on individual components and another based on their sum. We provide a model that simulates this type of behavior and is able to reproduce the ellipse orientations we found.

A midline azimuthal channel in human spatial hearing

Neurophysiological and psychophysical evidence has driven the formulation of a hemifield model of mammalian sound localization in which the perceived location of a stimulus is based on the relative activity of two hemifield-tuned azimuthal channels that are broadly responsive to contralateral auditory space and have overlapping medial borders. However, neurophysiological work in mammals has consistently found neurons selective for sound sources at the midline, which may indicate the existence of a third, ‘midline’, perceptual channel. In three experiments, the existence of three (left, right, midline) perceptual channels for azimuth in man was examined using auditory selective adaptation paradigms. If no midline channel exists, exposure to highly lateralized, symmetrical adaptator frequencies should not result in a shift in the perceived intracranial location of subsequent test tones away from the adaptors because the relative activation of the two hemifield channels will remain the same. Rather, our results indicate a shift in perceived test tones towards the azimuthal midline. This result can best be explained by a perceptual/neural channel tuned to sounds located along the midline. The present study gives the first psychophysical evidence of a midline channel serving human auditory localization, adding to the earlier evidence on the same point from animal neurophysiological studies.

Losing control

Neurologic examination after focal motor injury tends to focus on weakness rather than control. One reason for this may be the implicit assumption that weakness precludes control. Most neurologists, however, are familiar with the common bedside finding in patients with hemiparesis after stroke: they can squeeze your hand with surprising force but cannot make individuated finger movements. This dissociation is also seen when comparing the effect of a unilateral hemispheric stroke on motor performance in the ipsilateral arm; strength is unaffected but skilled movements are impaired.1 The separation between control of movement and of isometric force has a long tradition in the design of robot arms2 and psychophysical evidence suggests that these 2 types of control may be partitioned in the brain.3 In this issue of Neurology ®, van Hedel et al.4 shed further anatomic light on the strength/skill dissociation by comparing leg motor deficits in patients with incomplete spinal cord injury (iSCI) and patients with unilateral hemispheric stroke. The main hypothesis was that these 2 patient groups would show differential skill levels despite comparable degrees of weakness. The hypothesis was based on the idea that in iSCI, all descending pathways from the brain to the spinal cord segments below the lesion are affected in the …

Psychophysical evidence for long-range influence on foveal adaptation

Psychophysical evidence for long-range influence on foveal adaptation

Psychophysical evidence for the normalization model of attention

Psychophysical evidence for the normalization model of attention

Psychophysical evidence that top-down input effects error directions in a choice-response saccade task

Psychophysical evidence that top-down input effects error directions in a choice-response saccade task

Psychophysical evidence for abnormal magnocellular processing in 6-month olds infants with autism in their family

Previous psychophysical studies have shown impairments in motion processing, a dorsal visual stream function, in children with autism (e.g., Spencer et al., 2000). Since the dorsal stream receives input mainly from the magnocellular (M), and little from the parvocellular (P), pathway, these findings may reflect abnormal processing in the M pathway in autism. To test this abnormal M pathway hypothesis, and to determine whether such abnormalities are present early in life, we compared M and P pathway function in infants with an older sibling with autism (i.e., “at-risk” infants, who have a ∼10% chance of ultimately being diagnosed with autism, and added risk for sub-clinical social and communication deficits) with typical infants. M and P pathway function was assessed by obtaining luminance (L) and red/green chromatic (C) contrast sensitivities, respectively (see Dobkins et al., 1999). L and C sensitivities were determined in 88 typical and 11 at-risk 6-month-olds, using forced-choice preferential looking. The results of a 2-factor ANOVA (factor 1=subject group: at-risk vs. typical, factor 2=stimulus type: L vs. C) yielded a significant interaction ((F(1,97) = 5.4, p < 0.05). These results suggest abnormalities in the relative integrity of M vs. P pathways in at-risk infants. Specifically, differences were observed for L sensitivity (at-risk: mean log = 1.53, se = 0.07; typical: mean log = 1.38, se = 0.04), but not C sensitivity (at-risk: mean log = 1.51, se = 0.08; typical: mean log = 1.54, se = 0.03). Although L sensitivity in at-risk infants was actually enhanced, this nonetheless suggests abnormalities in M pathway processing, which could potentially serve as a phenotypic marker for autism, and may explain some of the cognitive/behavioral problems of individuals with autism. Preliminary data will be presented investigating correlations between these visual data at 6-months and social-communicative behaviors at 10-months, and diagnostic outcomes at 24-months.

Psychophysical evidence against the use of orientation disparity in the perception of slant.

Psychophysical evidence against the use of orientation disparity in the perception of slant.

Iso- and cross-orientation surround modulation of contrast detection and discrimination

Background: Neurophysiological and psychophysical evidence reveals strong modulation of contextual stimuli on visual performance. Here we studied how contrast detection and discrimination in foveal vision are affected by surround stimuli at iso- or cross-orientations. Methods: The target was a small D6 grating (8 cpd, vertical) centered on a slightly larger sinusoid grating pedestal (8 cpd, vertical, contrast = 0–40%) with an abutting sinusoid grating surround (8 cpd, contrast = 0–80%, horizontal or vertical). Thresholds were measured with a temporal 2AFC staircase procedure. Results: (1) Iso-oriented surrounds have little effect on detection except at high contrast where they become suppressive. For discrimination, iso-oriented surrounds facilitate when the surround/center contrast ratio is <= 1 and suppress when the s/c ratio is >1. (2) Surprisingly, cross-oriented surrounds can facilitate detection and discrimination by a factor of up to 2, even in the dipper region of the tvc function. This effect is not due to reduced uncertainty, since the slope of the psychometric function is unchanged by the surround, and facilitation is not impaired by jittering the surround contrast. Cross-orientation facilitation is largely unaffected by the s/c ratio. Under each surround contrast condition, facilitation dips when the pedestal contrast is around 5–20%, and becomes stronger at lower and higher pedestal contrasts. The data also indicate that cross-oriented surrounds may facilitate detection and near-threshold discrimination (pedestal contrast <10%) via direct exCitation, and facilitate suprathreshold discrimination (pedestal contrast >=10%) via indirect disinhibition of masking. Conclusion: Our results suggest that surround effects participate in complex gain control processes in spatial filters, which are not easily explained by the standard spatial filter models, but are supported by recent neurophysiological findings.

Disparity sensitivity in man and owl: Psychophysical evidence for equivalent perception of shape-from-stereo

The perception of shape-from-stereo is best characterized by the spatial disparity-contrast sensitivity function (DSF). This is the stereo analogue of the well-known luminance-contrast sensitivity function (CSF). In principle, the DSF and CSF portray a visual system's ability to detect spatial modulation as specified by changes in binocular disparity and luminance, respectively. In humans, less fine detail is visible in the stereo domain than is possible in the luminance domain. Here, we characterize for the first time the DSF in a non-human species, viz. the barn owl. At the same time, we re-examined the human DSF with identical apparatus and methods to directly compare between two vertebrate species that evolved stereovision independently. We discovered a close relationship between the owl and human ability to detect shape-from-stereo. In particular, the shift in absolute position between the human and owl DSF, as measured here, closely corresponds to the shift in absolute position between their respective CSFs, as known from the literature. In conclusion, our study establishes unprecedented experimental proof of a striking similarity in the prowess of humans and owls to achieve shape-from-stereo.

Plastic brain mechanisms for attaining auditory temporal order judgment proficiency

Accurate perception of the order of occurrence of sensory information is critical for the building up of coherent representations of the external world from ongoing flows of sensory inputs. While some psychophysical evidence reports that performance on temporal perception can improve, the underlying neural mechanisms remain unresolved. Using electrical neuroimaging analyses of auditory evoked potentials (AEPs), we identified the brain dynamics and mechanism supporting improvements in auditory temporal order judgment (TOJ) during the course of the first vs. latter half of the experiment. Training-induced changes in brain activity were first evident 43–76 ms post stimulus onset and followed from topographic, rather than pure strength, AEP modulations. Improvements in auditory TOJ accuracy thus followed from changes in the configuration of the underlying brain networks during the initial stages of sensory processing. Source estimations revealed an increase in the lateralization of initially bilateral posterior sylvian region (PSR) responses at the beginning of the experiment to left-hemisphere dominance at its end. Further supporting the critical role of left and right PSR in auditory TOJ proficiency, as the experiment progressed, responses in the left and right PSR went from being correlated to un-correlated. These collective findings provide insights on the neurophysiologic mechanism and plasticity of temporal processing of sounds and are consistent with models based on spike timing dependent plasticity.