Here, the difference in firing rate between two directions of motion is quite modest, and is most obvious in the later part of the response. (B) The temporal spiking response of a neuron in the macaque auditory cortex (A1) in response to a moving auditory stimulus. The vertical line indicates the preferred direction of motion, and the inset shows the mean spiking responses (with the spontaneous rate subtracted) in polar plot form, showing clear direction selectivity. (A) A typical visual direction tuning curve from a neuron in the marmoset visual cortex (area MT) in response to a moving dot stimulus (data from Chaplin et al., 2017). Encoding of direction of motion in the visual and auditory systems. For example, Figure 1A shows the response of a direction tuned neuron: the neuron shows strong responses to motion towards the upper left quadrant, and progressively weaker responses for directions further away.įigure 1.
Thus, direction selective neurons in the visual system can encode the direction of motion within their receptive fields. Specifically, the spiking (action potential) responses of neurons are tuned to the direction of moving stimuli, meaning that they are more active in response to a specific direction of motion compared to other directions ( Dubner and Zeki, 1971 Baker et al., 1981 Maunsell and Van Essen, 1983a Albright, 1984 Desimone and Ungerleider, 1986 Saito et al., 1986 Tanaka and Saito, 1989 Chaplin et al., 2017). However, the visual system goes one step further, with direction of motion being explicitly represented at the level of the single cell. Therefore, the responses of neurons in the visual system are inherently capable of coding the spatial location of visual stimuli, and in theory, could encode direction of motion by the sequential activation of populations of neurons with different receptive field locations. In the visual system, most neurons have spatially defined receptive fields, which are ultimately defined by inputs from specific regions of the retina. Spatial features are represented in fundamentally different ways in the visual and auditory systems. Encoding of Direction of Motion in the Activity of Cortical Neurons In this mini-review article, we will discuss the encoding of direction of motion in the visual and auditory systems, with emphasis on the cortical systems that are involved in translational motion, especially in azimuth (leftwards and rightwards motion), as this is the most common type of motion used in audiovisual integration studies.
In primates, the cerebral cortex contains a network of regions that are specialized for motion processing, but the systems for processing the motion of visual features and sounds are mediated by different brain regions, and underpinned by different physiological mechanisms. It is therefore not surprising that most, if not all brains have dedicated neural circuits for the perception of motion. The natural world abounds with motion, making this a highly salient cue to guide animals in interacting with the environment. We also discuss the way in which humans integrate of audio and visual motion cues, and the regions of the cortex that may mediate this process. We compare the representations of space and motion in the visual and auditory systems, and examine how single neurons in these two sensory systems encode the direction of motion. Here we, review the key cortical regions for motion processing, focussing on translational motion. A great deal of progress has been made in understanding how visual motion perception is governed by the activity of single neurons in the primate cerebral cortex, but far less progress has been made in understanding both auditory motion and audiovisual motion integration. Thus, it can highly advantageous to detect motion elicited from sensory signals of either modality, and to integrate them to produce more reliable motion perception. In the natural world, moving objects in the visual field often produce concurrent sounds. The ability of animals to detect motion is critical for survival, and errors or even delays in motion perception may prove costly. 2Australian Research Council (ARC) Centre of Excellence for Integrative Brain Function, Monash University Node, Clayton, VIC, Australia.1Neuroscience Program, Biomedicine Discovery Institute and Department of Physiology, Monash University, Clayton, VIC, Australia.
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