Temporal limits of long-range phase discrimination across the visual field

When two flickering sources are far enough apart to avoid low-level motion signals, phase judgment relies on the temporal individuation of the light and dark phases of each source. The highest rate at which the individuation can be maintained has been referred to as Gestalt flicker fusion [Van de Grind, W. A., Grüsser, O. -J., & Lunkenheimer, H. U. (1973). Temporal transfer properties of the afferent visual system. Psychophysical, neurophysiological and theoretical investigations. In R. Jung (Ed.), Handbook of sensory physiology (Vol. VII/3, pp. 431-573). Berlin: Springer, Chapter 7] and this has been taken as a measure of the temporal resolution of attention [Verstraten, F. A., Cavanagh, P., & Labianca, A. T. (2000). Limits of attentive tracking reveal temporal properties of attention. Vision Research, 40, 3651-3664; Battelli, L., Cavanagh, P., Intriligator, J., Tramo, M. J., Henaff, M. A., Michel, F., et al. (2001). Unilateral right parietal damage leads to bilateral deficit for high-level motion. Neuron, 32, 985-995]. Here we examine the variation of the temporal resolution of attention across the visual field using phase judgments of widely spaced pairs of flickering dots presented either in the upper or lower visual field and at either 4 degrees or 14 degrees eccentricity. We varied inter-dot separation to determine the spacing at which phase discriminations are no longer facilitated by low-level motion signals. Our data for these long-range phase judgments showed that temporal resolution decreases only slightly with increased distance from center of gaze (decrease from 11.4 to 8.9 Hz between 4 degrees to 14 degrees ), and does not differ between upper and lower visual fields. We conclude that the variation of the temporal limits of visual attention across the visual field differs markedly from that of the spatial resolution of attention.     

Aging and visual processing: declines in spatial not temporal integration

Age-related declines in vision are well documented in the literature. In the present study we examined whether changes in spatial or temporal integration contribute to this decline. Younger (mean age of 21) and older (mean age of 745) subjects were asked to identify 2D shapes based on kinetic occlusion information---the accretion and deletion of texture during motion. The results of the first experiment indicated age-related decrements in spatial but not temporal integration. In the second experiment we manipulated the lifetime of motion stimuli to more directly examine temporal integration. The results indicated no differential effect of age on temporal integration. The results considered together suggest age related changes in recovering 2D shape from occlusion are the result of spatial but not temporal integration. Age-related changes in neural inhibition and ACh for regulating spatial integration are proposed as possible mechanisms for this decline.     

Temporal integration following intensification of long-lasting visual displays

Duration of visible persistence is known to be inversely related to the duration of the inducing stimulus, within a critical interval estimated at between 100 and 150 msec. Stimuli longer than the critical interval yield little or no persistence. Six experiments investigated whether a brief period of intensification at the end of a stimulus longer than the critical interval could restore visible persistence. In the first experiment, a punctate stimulus ceased to give rise to visible persistence at exposure durations longer than the critical interval. The second experiment showed that persistence could be restored to a long display by briefly intensifying the component dots just before the end of the display. The remaining four experiments explored the limits and the distinguishing characteristics of this effect. Two alternative explanations of the results are described and evaluated.     

Temporal integration in cat visual cortex: a test of Bloch's law

Some units in the cat visual cortex fail to respond to a briefly flashed bar and it has been suggested that such neurons function as visual integrators with a long time constant. To test this integrator hypothesis, a study was made using presentations of a bar, flashed over the receptive field for various durations and at different luminances. Some cortical cells indeed showed an increase in the time to peak latency and in the response amplitude when stimulus duration was prolonged up to 320 msec. Such units obeyed Bloch's law for durations over 100 msec.     

Snakes and ladders: the role of temporal modulation in visual contour integration.

We investigated temporal aspects of the cortical mechanisms supporting visual contour integration by measuring observers' efficiency at detecting fragmented contours, composed of Gabor micropatterns, embedded in a field of distractor elements. Gabors consisted of a static Gaussian enveloping a sinusoidal carrier which was temporally modulated by motion or counter-phase flicker. The elements forming the path could be oriented either parallel ('snakes') or perpendicular to the contour orientation ('ladders'). Sensitivity to contour structure (estimated by measuring the maximum tolerable element orientation jitter supporting contour detection) was increased when the elements were drifting or flickering. Snakes were more detectable than ladders under all conditions. The increase in sensitivity conferred by drifting carriers was present even when the elements in the same stimulus were drifting at a range of speeds spanning almost three octaves. These results lend further support to the notion that the contour integration system receives separate transient and sustained input.     

On temporal hyperacuity in the human visual system

The spatial grain of the human visual system has always been a central topic for visual sciences, and the optical and physiological basis of perceptual limitations are well described. In particular, we have thorough accounts of spatial hyperacuity, which refers to a precision in the spatial localisation of stimulus contours that is better than the photoreceptor grain that determines spatial resolution. However, although the temporal resolution of the human visual system is comparably well described, we have almost no direct knowledge about the precision of localising visual stimuli in time in the absence of correlated spatial cues. The present study addresses this question by comparing directly the temporal resolution of human observers with their temporal acuity as measured in a temporal bisection task. Despite some improvement with practice, temporal acuity in this task does not fall below 20-30 ms in the best case, which is similar to the temporal resolution limit, and performance does not improve for comparison tasks with multiple stimulus presentations. The absence of visual hyperacuity for purely temporal modulations as tested here contrasts with processing limitations for other types of visual information in comparable tasks, and with other sensory modalities, in particular to those of the auditory system. Such differences can be interpreted in the context of the ecological requirements for organising behaviour, and the functional design of nervous systems.     

Sensory integration across modalities: how kinaesthesia integrates with vision in visual orientation discrimination

Stimuli in one modality can affect the appearance and discriminability of stimuli in another, but how they do so is not well understood. Here we propose a theory of the integration of sensory information across modalities. This is based on criterion setting theory (CST; Treisman and Williams, 1984), an extension of signal detection theory which models the setting and adjustment of decision criteria. The theory of sensory integration based on CST (CST-SI) offers an account of cross-modal effects on sensory decision-making; here we consider its application to orientation anisotropy. In this case, CST-SI postulates that the postural senses are concerned with the relations between momentary body posture and the cardinal dimensions of space, vertical and horizontal, and that they also contribute to stabilizing perception of the cardinal orientations in vision through actions on the corresponding visual decision criteria, but that they have little effect on perception of diagonal orientations. Predictions from CST-SI are tested by experimentally separating the contributions that different information sources make to stabilizing the visual criteria. It is shown that reducing relevant kinaesthetic input may increase the variance for discrimination of the visual cardinal axes but not the obliques. Predictions that shift in the location of the psychometric function would be induced by varying the distribution of the test stimuli, and that this effect would be greater for oblique than cardinal axes were confirmed. In addition, peripheral visual stimuli were shown to affect the discrimination of cardinal but not oblique orientations at the focus of vision. These results support the present account of anisotropies.     

Temporal properties of the human visual nervous system

An image stabilisation system was used to study basic temporal properties which are normally obscured by eye movements. The time course of visual response was measured for three types of presentation of a near-threshold line—sudden presentation (on-response), switching it off again (which generates a negative afterimage) and a sudden small lateral displacement. A general model can be used to fit all these data, indicating that the responses to all three stimuli are generated by similar mechanisms. The responses are similar to that of a capacitor-coupled amplifier with a time constant of about 2.5 sec; the corresponding prediction that the visual frequency response should extend down to about 1/6 Hz was confirmed by measuring contrast thresholds for a line oscillated sinusoidally through a small amplitude. The relation of these observations to the process of light adaptation is discussed.     

Temporal integration in diseased eyes. I. Exposure duration in visual acuity testing

Critical duration in visual acuity testing can be viewed as an expression of temporal integration in the human visual system. We examined this phenomenon in 13 eyes with central serous retinopathy (CSR) and 6 eyes with macular edema, by measuring visual acuity at several limited exposure times. The results were then compared with those for 17 normal eyes. The acuity target was a single Landolt ring projected upon a small square screen. The size, direction, and exposure time of the target were computer controlled. The mean critical durations of the CSR and macular edema groups were 1.78 sec. and 2.69 sec. respectively. These values were significantly (p less than 0.01) longer than the mean critical duration of the normal control group (0.62 sec.). Although the mechanism behind the longer critical duration in diseased eyes remains poorly understood, we believe this method provides a possible approach to the study of diseased visual conditions.     

Spatial integration in the crustacean visual system: peripheral and central sources of non-linear summation

The spatial summing characteristics of the crustacean sustaining fiber were examined with concentric stimuli and observed to be highly dependent upon the stimulus intensity. In the dark adapted condition and with response criteria at or near the central threshold, summation is approximately linear. With moderate light adaptation or relatively large response criteria, spatial summation becomes markedly nonlinear. The nonlinearities appear as a diminishing sensitivity with decreasing stimulus diameters. These results are interpreted in terms of the nonlinear intensity-voltage transform of the retinula cell. An additional source of nonlinearity was observed under suprathreshold conditions with stimulus dia in excess of 10–12°. A two-point stimulus procedure indicated that these results are consistent with a lateral inhibitory mechanism operating within the excitatory field.     

Temporal frequency discrimination in human vision: evidence for an additional mechanism in the low spatial and high temporal frequency region

Temporal frequency discrimination at and above the detection threshold has been studied using gratings of low (0.2 c/deg) and medium (2 c/deg) spatial frequencies. At 2 c/deg the results of previous investigators are confirmed: The results being consistent with the existence of two broadly tuned and directionally selective temporal mechanisms (up to 32 Hz). For the lower spatial frequency an additional temporal frequency discrimination at threshold can be made between 4 and 32 Hz and enhanced temporal frequency discrimination at suprathreshold levels occurs above 24 Hz. One interpretation of this result is the existence of one or more additional temporal mechanisms with restricted spatial acuity responding to higher temporal frequencies.     

The influence of level and polarity of figure-ground contrast on vision.

PURPOSE: To document the effect upon human foveal vision of changes in the level and polarity of figure-ground contrast under photopic controlled test conditions, with particular emphasis on performance at low contrast levels. METHODS: Using a forced-choice psychophysical paradigm, threshold acuity estimates were derived at 9 discrete levels over a near-3 octave contrast range for Landolt ring-type stimuli of either positive or negative polarity. Data were obtained under binocular conditions from 10 young adults, each wearing their optimum low myopic spectacle correction. RESULTS: Visual acuity declined linearly with reducing stimulus contrast, the deterioration increasing substantially at <10% figure-ground contrast regardless of stimulus polarity. Performance was slightly (but not statistically significantly) better for positive contrast stimuli. CONCLUSION: Irrespective of contrast polarity, a reduction in stimulus figure-ground contrast <10% produces an accelerated decrement in photopic foveal vision compared to the performance at levels >10%. Some clinical and practical implications of this outcome are considered with regard to the examination of patients with normal and compromised visual function.     

Temporal integration in diseased eyes. III. Fixation movements in visual acuity testing]

The fixation movements which occur during visual acuity testing were observed in order to clarify the mechanism behind the critical duration in visual acuity testing, which we found in a previous study to be significantly longer in central serous retinopathy and macular edema. Photoelectric oculography (p-EOG) was used to record horizontal eye movement in this study. In normal eyes the frequency and amplitude of the microsaccades were smaller while the acuity target was shown than when it was not shown. This phenomenon was also observed in eyes with macular edema. These results suggest that microsaccades play no positive role in the reading of visual acuity targets, and that they bear little relation to the phenomenon of critical duration in visual acuity testing.     

Attribute-invariant orientation discrimination at an early stage of processing in the human visual system

This study investigated event-related brain potentials (ERPs) during selective attention to the orientation of a bar comprised of two squares, which were defined by only color or motion (intra-attribute conditions) or both (interattribute condition). An early positive potential in association with orientation selection was elicited for all conditions in similar latency ranges but with different scalp distributions. These results suggest that attribute-invariant orientations can be discriminated at an early stage of processing in the human brain, which fills a gap between monkey electrophysiology and human psychophysics, while attribute-specific orientations are also available in a given context.     

Daily rhythm of vigilance assessed by temporal resolution of the visual system

To assess the daily distribution of temporal resolution in visual detection, binocular double-pulse resolution (DPR) was measured over a 24 h period in six healthy subjects. DPR showed a significant daily variation with an amplitude for the foveal stimulus of up to 60%. Like in other vigilance-dependent daily rhythms, optimal performance occurred around midday. The DPR measurements described here are an excellent method for assessment of vigilance and mental alertness (e.g. in pharmacological studies). They show strong time-of-day differences, are highly reliable across successive measurements, and can be fully automated.     

Ocular integration in the human visual cortex

Human striate cortex contains an orderly map of the contralateral visual field, which is distorted to make a disproportionate amount of tissue available for the representation of the macula. Engrafted on the retinotopic map is a system of alternating inputs known as ocular dominance columns. These columns consist of interleaved bands of geniculocortical afferents in layer 4C serving either the right eye or the left eye. They can be revealed in humans with a history of prior visual loss in one eye by processing striate cortex for cytochrome oxidase at autopsy. Because their geniculate input is segregated, cells within ocular dominance columns in layer 4C respond to stimulation of one eye only. These monocular cells converge onto binocular cells in other layers, integrating signals from the two eyes. The columns in humans appear similar to those found in many primate species, including the macaque. In the squirrel monkey, however, the occurrence of ocular dominance columns is highly variable. Some squirrel monkeys lack columns, yet they seem to have no impairment of visual function. In animals with weakly expressed columns, one can detect a cortical pattern of metabolic activity corresponding to retinal blood vessels. It appears because visual deprivation from shadows cast by blood vessels induces remodeling of geniculocortical afferents, in a manner akin to the shrinkage of ocular dominance columns from congenital cataract. Although the function of ocular dominance columns is unknown, their metabolism is altered in strabismus, suggesting a role in visual suppression.