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Title
Abstract
Coarse-To-Fine Strategy of Visual Analysis
Concluding Remarks and Future Perspectives
References
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Psychology

Temporal Coding of Visual Space

Profile image of Ehud AhissarEhud Ahissar

2018, Trends in Cognitive Sciences

https://doi.org/10.1016/J.TICS.2018.07.009
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13 pages

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Abstract

Establishing a representation of space is a major goal of sensory systems. Spatial information, however, is not always explicit in the incoming sensory signals. In most modalities it needs to be actively extracted from cues embedded in the temporal flow of receptor activation. Vision, on the other hand, starts with a sophisticated optical imaging system that explicitly preserves spatial information on the retina. This may lead to the assumption that vision is predominantly a spatial process: all that is needed is to transmit the retinal image to the cortex, like uploading a digital photograph, to establish a spatial map of the world. However, this deceptively simple analogy is inconsistent with theoretical models and experiments that study visual processing in the context of normal motor behavior. We argue here that, as with other senses, vision relies heavily on temporal strategies and temporal neural codes to extract and represent spatial information. Stable Visual Representations, but a Moving Visual Image Like a camera, the eye forms an image of the external scene on its posterior surface where the retina is located, with its dense mosaic of photoreceptor cells (see Glossary) that convert light into electrochemical signals. At each moment in time, all spatial information is present in the visual signals striking the photoreceptors, which explicitly encode space by their position within the retinal array. This camera model of the eye and the spatial coding idea have long dominated visual neuroscience. Although the specific reference frames of spatial representations (e.g., retinotopic vs spatiotopic) have been intensely debated [1,2], spatial information has always been assumed to originate from the receptor layout in the retina. Alas, the eye does not behave like a camera. While a photographer usually takes great care to ensure that the camera does not move, the eyes insist on moving continuously [3-6]. Humans perform rapid gaze shifts, known as saccades, 2-3 times per second. Even though models of the visual system often assume that the visual input is a stationary image during fixational pauses between successive saccades, small eye movements, known as fixational eye movements, continually occur. These movements displace the stimulus by considerable amounts on the retina, therefore continually changing the light signals striking the photoreceptors [7,8] (Box 1). Furthermore, unlike the film in a camera, the visual system depends on temporal transients. Neurons in the retina, thalamus, and later stages of the visual pathways respond strongly to temporal changes [9-13]. Visual percepts tend to fade away in the complete absence of temporal transients [14-17], and spatial changes that occur too slowly are not even detected by humans [18,19]. These considerations do not appear compatible with the standard idea that space is encoded solely by the position of neurons within spatial maps. They suggest that the visual system combines spatial sampling with temporal processing to extract and encode spatial information.

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    Yuji Iwasaka

    Frontiers in Neuroscience, 2020

    Rhythm tapping tasks are often used to explore temporal reproduction abilities. Many studies utilizing rhythm tapping tasks are conducted to evaluate temporal processing abilities with neurological impairments and neurodegenerative disorders. Among sensorimotor and cognitive disorders, rhythm processing abilities in constructional apraxia, a deficit in achieving visuospatial constructional activities, has not been evaluated. This study aimed to examine the rhythm tapping ability of patients with constructional apraxia after a stroke. Twenty-four patients were divided into two groups: with and without constructional apraxia. There were 11 participants in the constructional apraxia group and 13 in the without constructional apraxia group. The synchronization-continuation paradigm was employed in which a person performs a synchronized tapping activity to a metronome beat and continues tapping after the beat has stopped. For statistical analysis, a three-way mixed analysis of variance (...

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    Fixation-related saccadic inhibition in free viewing in response to stimulus saliency
    Yoram Bonneh

    Scientific Reports

    Microsaccades that occur during fixation were studied extensively in response to transient stimuli, showing a typical inhibition (Oculomotor Inhibition, OMI), and a later release with a latency that depends on stimulus saliency, attention, and expectations. Here, we investigated the hypothesis that in free viewing every saccade provides a new transient stimulation that should result in a stimulus-dependent OMI like a flashed presentation during fixation. Participants (N = 16) freely inspected static displays of randomly oriented Gabor texture images, with varied contrast and spatial frequency (SF) for periods of 10 s each. Eye tracking recordings were divided into epochs triggered by saccade landing (> 1 dva), and microsaccade latency relative to fixation onset was computed (msRT). We found that the msRT in free viewing was shorter for more salient stimuli (higher contrast or lower SF), as previously found for flashed stimuli. It increased with saccade size and decreased across s...

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    Visual feature tuning of superior colliculus neural reafferent responses after fixational microsaccades
    Fatemeh Khademi

    2019

    The primate superior colliculus (SC) is causally involved in microsaccade generation. Moreover, visually-responsive SC neurons across this structure’s topographic map, even at peripheral eccentricities much larger than the tiny microsaccade amplitudes, exhibit significant modulations of evoked response sensitivity when stimuli appear peri-microsaccadically. However, during natural viewing, visual stimuli are normally stably present in the environment and are only shifted on the retina by eye movements. Here we investigated this scenario for the case of microsaccades, asking whether and how SC neurons respond to microsaccade-induced image jitter. We recorded neural activity from two male rhesus macaque monkeys. Within the response field (RF) of a neuron, there was a stable stimulus consisting of a grating of one of three possible spatial frequencies. The grating was stable on the display, but microsaccades periodically jittered the retinotopic RF location over it. We observed clear s...

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    Contrast sensitivity reveals an oculomotor strategy for temporally encoding space
    Jonathan Victor

    eLife, 2019

    The contrast sensitivity function (CSF), how sensitivity varies with the frequency of the stimulus, is a fundamental assessment of visual performance. The CSF is generally assumed to be determined by low-level sensory processes. However, the spatial sensitivities of neurons in the early visual pathways, as measured in experiments with immobilized eyes, diverge from psychophysical CSF measurements in primates. Under natural viewing conditions, as in typical psychophysical measurements, humans continually move their eyes even when looking at a fixed point. Here, we show that the resulting transformation of the spatial scene into temporal modulations on the retina constitutes a processing stage that reconciles human CSF and the response characteristics of retinal ganglion cells under a broad range of conditions. Our findings suggest a fundamental integration between perception and action: eye movements work synergistically with the spatio-temporal sensitivities of retinal neurons to en...

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    One-year-later spontaneous EEG features predict visual exploratory human phenotypes
    Ilaria Mazzonetto

    Communications Biology

    During visual exploration, eye movements are controlled by multiple stimulus- and goal-driven factors. We recently showed that the dynamics of eye movements –how/when the eye move– during natural scenes’ free viewing were similar across individuals and identified two viewing styles: static and dynamic, characterized respectively by longer or shorter fixations. Interestingly, these styles could be revealed at rest, in the absence of any visual stimulus. This result supports a role of intrinsic activity in eye movement dynamics. Here we hypothesize that these two viewing styles correspond to different spontaneous patterns of brain activity. One year after the behavioural experiments, static and dynamic viewers were called back to the lab to record high density EEG activity during eyes open and eyes closed. Static viewers show higher cortical inhibition, slower individual alpha frequency peak, and longer memory of alpha oscillations. The opposite holds for dynamic viewers. We conclude ...

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