Is the visual field temporally homogeneous?

fMRI representational similarity analysis reveals graded preferences for chromatic and achromatic stimulus contrast across human visual cortex

Human visual cortex is partitioned into different functional areas that, from lower to higher, become increasingly selective and responsive to complex feature dimensions. Here we use a Representational Similarity Analysis (RSA) of fMRI-BOLD signals to make quantitative comparisons across LGN and multiple visual areas of the low-level stimulus information encoded in the patterns of voxel responses. Our stimulus set was picked to target the four functionally distinct subcortical channels that input visual cortex from the LGN: two achromatic sinewave stimuli that favor the responses of the high-temporal magnocellular and high-spatial parvocellular pathways, respectively, and two chromatic stimuli isolating the L/M-cone opponent and S-cone opponent pathways, respectively. Each stimulus type had three spatial extents to sample both foveal and para-central visual field. With the RSA, we compare quantitatively the response specializations for individual stimuli and combinations of stimuli in each area and how these change across visual cortex. First, our results replicate the known response preferences for motion/flicker in the dorsal visual areas. In addition, we identify two distinct gradients along the ventral visual stream. In the early visual areas (V1-V3), the strongest differential representation is for the achromatic high spatial frequency stimuli, suitable for form vision, and a very weak differentiation of chromatic versus achromatic contrast. Emerging in ventral occipital areas (V4, VO1 and VO2), however, is an increasingly strong separation of the responses to chromatic versus achromatic contrast and a decline in the high spatial frequency representation. These gradients provide new insight into how visual information is transformed across the visual cortex.

Temporal information loss in the macaque early visual system

Stimuli that modulate neuronal activity are not always detectable, indicating a loss of information between the modulated neurons and perception. To identify where in the macaque visual system information about periodic light modulations is lost, signal-to-noise ratios were compared across simulated cone photoreceptors, lateral geniculate nucleus (LGN) neurons, and perceptual judgements. Stimuli were drifting, threshold-contrast Gabor patterns on a photopic background. The sensitivity of LGN neurons, extrapolated to populations, was similar to the monkeys' at low temporal frequencies. At high temporal frequencies, LGN sensitivity exceeded the monkeys' and approached the upper bound set by cone photocurrents. These results confirm a loss of high-frequency information downstream of the LGN. However, this loss accounted for only about 5% of the total. Phototransduction accounted for essentially all of the rest. Together, these results show that low temporal frequency information is lost primarily between the cones and the LGN, whereas high-frequency information is lost primarily within the cones, with a small additional loss downstream of the LGN.

Prolonged temporal interaction for peripheral visual processing in schizophrenia: evidence from a three-flash illusion

BACKGROUND

Coherent perception of the visual world requires orderly processing of spatially and temporally distributed visual information across the visual field. The organization of this visual information is impaired in schizophrenia. We previously found that visual temporal integration in patients is prolonged, using flashes presented to the central fovea. In this study, we investigated this temporal interaction in both the fovea and fairly far out in the peripheral visual field.

METHODS

We used a 'three-flash' illusion paradigm in which two spatially-coincident light pulses (of 1 ms each) are perceived by healthy individuals as one, two or three flashes depending on the time interval between the pulses. In each trial, two light pulses were presented in the fovea or 34° out in the right visual field. The inter-stimulus pulse interval (ISI) ranged from 30 to 310 ms. The task for patients (n=28) and controls (n=26) was to indicate the number of flashes (one, two or three) perceived after each two-pulse presentation.

RESULTS

For the controls, the peak of the three-flash illusion was shifted to longer ISIs (150 ms) in the periphery compared to the fovea (110 ms). For the patients, the three-flash illusion was greater and occurred at longer ISIs (270 ms in the periphery and 190 ms at the fovea).

CONCLUSION

Compared to the central visual field, the range of temporal interactions in the periphery is prolonged to a greater extent in schizophrenia. This exacerbated temporal expansion in peripheral vision suggests a coarse temporal resolution for visual and cognitive organization in this mental disorder.

Local visual energy mechanisms revealed by detection of global patterns

A central goal of visual neuroscience is to relate the selectivity of individual neurons to perceptual judgments, such as detection of a visual pattern at low contrast or in noise. Since neurons in early areas of visual cortex carry information only about a local patch of the image, detection of global patterns must entail spatial pooling over many such neurons. Physiological methods provide access to local detection mechanisms at the single-neuron level but do not reveal how neural responses are combined to determine the perceptual decision. Behavioral methods provide access to perceptual judgments of a global stimulus but typically do not reveal the selectivity of the individual neurons underlying detection. Here we show how the existence of a nonlinearity in spatial pooling does allow properties of these early mechanisms to be estimated from behavioral responses to global stimuli. As an example, we consider detection of large-field sinusoidal gratings in noise. Based on human behavioral data, we estimate the length and width tuning of the local detection mechanisms and show that it is roughly consistent with the tuning of individual neurons in primary visual cortex of primate. We also show that a local energy model of pooling based on these estimated receptive fields is much more predictive of human judgments than competing models, such as probability summation. In addition to revealing underlying properties of early detection and spatial integration mechanisms in human cortex, our findings open a window on new methods for relating system-level perceptual judgments to neuron-level processing.

Spatio-temporal templates of transient attention revealed by classification images

Visual attention is captured by transient signals in the periphery of the visual field, allowing enhanced perceptual representations in spatial tasks. However, it has been reported that the same cues impair performance in temporal tasks (e.g., Yeshurun, 2004; Yeshurun & Levy, 2003). This findings suggest that transient attention enhances the activity of slow, high-resolution channels, like parvocellular neurons, and/or shuts off faster channels better sensitive to low spatial frequencies, such as the ones of the magnocellular system. To test this idea, we have measured the spatio-temporal perceptive fields for transiently cued signals at various eccentricities using the classification images (CI) technique. At near eccentricities transient attention caused the perceptual templates to be sharper in space and characterized by much stronger high spatial frequency components. At the same time, they show a consistently larger temporal integration window. These effects of attention on perceptual filters are strongly reduced at far eccentricities and disappear when using longer target-cue lags. These data provide evidence in support of the parvocellular model of transient, exogenous attention, showing that in the presence of a well timed spatial cue observers rely on noisy evidence lasting longer and with finer spatial configurations.

Does cognitive perception have access to brief temporal events?

To determine whether conscious perception has access to brief temporal event, we asked subjects in an odd-man out paradigm to determine which of the four Gaussian blobs was flickering asynchronously in time. We measure synchrony thresholds as a function of the base temporal frequency for spatially scaled stimuli in foveal and peripheral vision. The results are consistent with a time delay of around 67 milliseconds (ms) for foveal vision and 91 ms for peripheral vision. We conclude that conscious perception has access to only relatively long (∼67 ms) time events.

Transient spatial attention degrades temporal resolution

To better understand the interplay between the temporal and spatial components of visual perception, we studied the effects of transient spatial attention on temporal resolution. Given that spatial attention sharpens spatial resolution, can it also affect temporal resolution? To assess temporal resolution, we measured the two-flash fusion threshold When two flashes of light are presented successively to the same location, the two-flash fusion threshold is the minimal interval between the flashes at which they are still perceived as two flashes, rather than a single flash. This assessment of temporal resolution was combined with peripheral precuing--a direct manipulation of transient spatial attention. This allowed us to demonstrate, for the first time, that spatial attention can indeed affect temporal resolution. However, in contrast to its effect on spatial resolution, spatial attention degrades temporal resolution. Two attentional mechanisms that could account for both attentional effects--enhanced spatial resolution and reduced temporal resolution--are discussed.

Temporal detection in human vision: dependence on eccentricity

Studies of human perception of time-varying luminance often aim to estimate either temporal impulse response shapes or temporal modulation transfer functions (MTFs) of putative temporal processing mechanisms. Previously, temporal masking data have been used to estimate the properties and numbers of these temporal mechanisms in central vision for 1 cycle per degree (cpd) targets [Fredericksen and Hess (1998)]. The same methods have been used to explore how these properties change with stimulus energy [Fredericksen and Hess (1997)] and spatial frequency [Fredericksen and Hess (1999)]. We present here analyses of the properties of temporal mechanisms that detect temporal variations of luminance in peripheral vision. The results indicate that a two-filter model provides the best model for our masking data, but that no multiple filter model provides an acceptable fit across the range of parameters varied in this study. Single-filter modelling shows differences between processing mechanisms at 1 cpd in central vision and those that operate eccentrically. We find evidence that this change is because of differences in relative sensitivities of the mechanisms, and to differences in fundamental mechanism impulse responses.

Role of onset asynchrony in contour integration

Evidence that visual grouping is facilitated when elements comprising a foreground figure are presented simultaneously, and are temporally separated from elements comprising the background, has suggested cortical synchronous oscillations as a possible neural substrate. Supporting this theory, Usher and Donnelly (Nature 394 (1998) 179) showed in one of their experiments that contour integration is facilitated when path and background elements alternate with an asynchrony below the integration time of the visual system, suggesting that these flickering stimuli interact with this hypothetical binding mechanism. I replicated this experiment and report that the effect depends in fact on the order of asynchrony between path and background elements in the first cycle of stimuli presented for more than 100 ms: facilitation in visual grouping only occurs when path elements are presented before background elements. A second experiment, exploring the effect of onset delays between path and background elements, demonstrates a strong priming effect of path elements. I conclude that Usher and Donnelly's result is likely due to the high sensitivity of the visual system to stimulus onset, and that simple flickering stimuli are inadequate for revealing the neural code for binding in figure-ground segregation without controlling for the effect of stimulus onset.

Is the rod visual field temporally homogeneous?

Cone vision has been shown to be temporally inhomogeneous across the visual field. In the periphery, contrast sensitivity is lower for low temporal frequencies and higher for high temporal frequencies. Here we ask a similar question for rod vision at mesopic luminances. Isolation is obtained by testing a well documented rod monochromat. We show that the rod visual field exhibits only a modest degree of temporal inhomogeneity.

Functional architecture of primate cone and rod axons

The cone axon is nearly four times thicker than the rod axon (1.6 vs 0.45 microns diameter). To assess how signal transfer and integration at the terminal depend on cable dimensions, a transducer (cone = ohmic conductance, rod = current source) coupled via passive cable to a sphere with a chloride conductance (representing GABAA receptor) was modelled. For a small signal in peripheral cone with a short axon, steady photosignal transfers independently of axon diameter despite a significant chloride conductance at the cone terminal. Temporally varying photosignal also transfers independently of axon diameter up to 20 Hz and is attenuated only 20% at 50 Hz. Thus, to accomplish the basic electrical functions of a peripheral cone, a thin axon would suffice. For a foveal cone with a long axon steady photosignal transfers independently of axon diameter, but temporally varying photosignal is attenuated 5-fold at 50 Hz for a thick axon and 10-fold for a thin axon. This might contribute to the lower sensitivity of central retina to high temporal frequencies. The cone axon contains 14-fold more microtubules than the rod axon, and its terminal contains at least 20-fold more ribbon synapses than the rod's. Since ribbon synapses sustain high rates of exocytosis, the additional microtubules (which require a thicker axon) may be needed to support a greater flux of synaptic vesicle components.

Losses in peripheral colour sensitivity predicted from "hit and miss" post-receptoral cone connections

On the basis of the early primate neurophysiological recordings, it was thought that the different cone types of the primate retina project selectively into the centre and surround of the receptive fields of cone opponent neurons, and more recently this view has been reasserted on the basis of physiological results. An alternative idea is that these projections are in fact unselective for cone type, and, therefore, cone opponency arises from chance variations in the proportions of different cone types in centre and surround. The issue is presently controversial with anatomical or physiological support for both hypotheses. Our results show that there is a selective loss of red-green colour sensitivity across the human visual field. Furthermore, this selective loss occurs under low temporal frequency conditions (0.5 Hz) which were selected to favour the mediation of both colour and luminance detection by a common P cell pathway and to exclude an M cell contribution to detection threshold. We show that "hit and miss" post-receptoral cone projections will produce a decline in cone opponency that is sufficient to account for this selective loss, thus providing psychophysical evidence consistent with this hypothesis.

The processing of temporal modulation at different levels of retinal illuminance

How does our temporal vision change as the mean illuminance reduces? We have examined the processing of near-threshold temporal information for a range of illuminance values (2850--0.15 phot td). At high illuminance, the modulation transfer function can be shown to be mediated via three underlying temporal filters that vary in sensitivity with spatial frequency. As the mean illuminance decreases these channels appear to change their sensitivity. Even at the lowest (scotopic) illuminance levels we were able to find evidence for at least two channels mediating detection threshold. There are also changes in the tuning properties of these channels such that the processing of high temporal frequencies is differentially compromised, resulting in a reduction in the flicker fusion limit of each channel, and a shift in the peak of the band-pass channel. The slope of the fall-off in sensitivity at high temporal frequencies is unaffected by test spatial frequency at each illuminance level, suggesting its limiting factor is one that is insensitive to spatial frequency. We propose that the changes in the tuning of the temporal filters occur because of an early (e.g. photoreceptor) change in the response dynamics, or by interactions between photoreceptors, rather than changes at or beyond the level of the channel response.

Effects of luminance and external temporal noise on flicker sensitivity as a function of stimulus size at various eccentricities

We studied how the dependence of flicker sensitivity on stimulus size was affected by the eccentricity of the stimulus at high luminance, at low luminance (with quantal noise), and at high luminance with the addition of pure white temporal noise. Flicker sensitivity was measured as a function of stimulus size for temporal frequencies of 1-30 Hz with uniform sinusoidally flickering spots. Sensitivity first increased with increasing stimulus size but then the increase saturated. At high luminance the saturation took place at larger stimulus sizes with increasing eccentricity. Without externally added temporal noise the maximum sensitivity was higher at the fovea than in the periphery at temporal frequencies of 1-10 Hz, but at 30 Hz this situation reversed. Therefore only the ascending parts of the spatial integration curves from various eccentricities could be superimposed by size scaling. E2, the eccentricity at which the spatial scale doubles, was found to be 2.2-2.7 deg for 1-10 Hz but 4.4 deg for 30 Hz. When enough temporal noise was added, performance at all stimulus sizes studied could be made independent of eccentricity by spatial scaling, since noise reduced maximum sensitivities to a constant level at all eccentricities. E2 was found to be 4.1 deg for 3 Hz and 7.2 deg for 30 Hz. When light level was reduced by 3 log10 units, foveal and peripheral flicker sensitivity functions almost superimposed at all stimulus sizes studied. Hence, at 1 and 3 Hz E2 was very large, about 70 and 22 deg, respectively. At 10 and 30 Hz no size scaling was needed and E2 was therefore infinite.

Suprathreshold temporal-frequency discrimination in the fovea and the periphery

To address the question of whether temporal-frequency information in the fovea and the periphery is processed in fundamentally different ways we measured temporal-frequency-discrimination thresholds for spatiotemporally narrow-band stimuli presented at suprathreshold contrast. Temporal-frequency-discrimination thresholds are similar (within a factor of 2) at the fovea and at 30 degrees in the periphery. We use a line-element approach and three spatiotemporally separable temporal mechanisms to model foveal and peripheral data with the same degree of fidelity. These findings suggest that not only are the front-end temporal mechanisms in the fovea and periphery likely to be similar but also the way in which their outputs are combined at more central sites is the same.

Perceived contrast as a function of adaptation duration

We measured how the perceived contrast of a sinusoidal grating fades as a function of time. Measurements were made for a range of temporal and spatial frequencies and eccentricities. Patterns of high temporal and low spatial frequency exhibited a greater and more rapid loss of apparent contrast (fade) than those of medium frequencies. The rate and amount of fading for a subgroup of moderate frequencies increased when presented peripherally rather than foveally. Further measurements revealed that gratings of disparate spatial frequencies, but with the same threshold sensitivity, exhibit very different fading characteristics but equal threshold elevation. We conclude that the differential loss of apparent contrast is not an artefact of differing proximities to threshold, nor can it be accounted for by differences in the adaptability of underlying spatio-temporal mechanisms at threshold. The differences in fading may thus reflect either a difference in the adaptability of underlying channels above threshold or a differential contribution of such channels to perceived contrast.

Temporal properties of letter identification in retinitis pigmentosa

The effect of stimulus duration on the visual acuity of individuals with retinitis pigmentosa (RP) was assessed by using Sloan letters of 100% contrast and durations ranging from 15 ms to 3.8 s. In addition, contrast thresholds for identifying Sloan letters (0.7 log minimum angle of resolution; 20/100 Snellen equivalent) were measured over the same range of durations in the same subjects with RP. Compared with results from a control group of subjects with normal vision, the subjects with RP showed losses in visual acuity at all stimulus durations, with a slightly though significantly greater reduction in visual acuity at short durations. The letter-contrast thresholds of the subjects with RP were elevated above the normal range to the same degree at all durations. Analysis of the results in the format of letter contrast sensitivity functions indicates that temporal summation for letter identification was normal for these subjects with RP and that their relatively greater acuity loss at short exposure durations was most likely related to their elevated letter contrast thresholds.