Surround suppression

Surround suppression is where the relative firing rate of a neuron may under certain conditions decrease when a particular stimulus is enlarged. It has been observed in electrophysiology studies of the brain and has been noted in many sensory neurons, most notably in the early visual system. Surround suppression is defined as a reduction in the activity of a neuron in response to a stimulus outside its classical receptive field. Surround suppression is where the relative firing rate of a neuron may under certain conditions decrease when a particular stimulus is enlarged. It has been observed in electrophysiology studies of the brain and has been noted in many sensory neurons, most notably in the early visual system. Surround suppression is defined as a reduction in the activity of a neuron in response to a stimulus outside its classical receptive field. The necessary functional connections with other neurons influenced by stimulation outside a particular area and by dynamic processes in general, and the absence of a theoretical description of a system state to be treated as a baseline, deprive the term 'classical receptive field' of functional meaning. The descriptor 'surround suppression' suffers from a similar problem, as the activities of neurons in the 'surround' of the 'classical receptive field are similarly determined by connectivities and processes involving neurons beyond it.) This nonlinear effect is one of many that reveals the complexity of biological sensory systems, and the connections of properties of neurons that may cause this effect (or its opposite) are still being studied. The characteristics, mechanisms, and perceptual consequences of this phenomenon are of interest to many communities, including neurobiology, computational neuroscience, psychology, and computer vision. The classical model of early vision presumes that each neuron responds independently to a specific stimulus in a localized area of the visual field. (According to Carandini et al (2005), this computational model, which may be fit to various datasets, 'degrade quickly if we change almost any aspect of the test stimulus.') The stimulus and corresponding location in the visual field are collectively called the classical receptive field. However, not all effects can be explained by via ad hoc independent filters. Surround suppression is one of an infinite number of possible effects in which neurons do not behave according to the classical model. These effects are collectively called non-classical receptive field effects, and have recently become a substantial research area in vision and other sensory systems. During surround suppression, neurons are inhibited by a stimulus outside their classical receptive field, in an area loosely termed deemed the 'surround.' Electrophysiology studies are used to characterize the surround suppression effect. Vision researchers that record neural activity in the primary visual cortex (V1) have seen that spike rates, or neural responses, can be suppressed in as many as 90% of neurons by stimuli outside of their surround. In these cells, the spike rates may be reduced by as much as 70%. The suppressive effect is often dependent on the contrast, orientation, and direction of motion of the stimulus stimulating the surround. These properties are highly dependent on the brain area and the individual neuron being studied. In MT, for instance, cells can be sensitive to the direction and velocity of stimuli up to 50 to 100 times the area of their classical receptive fields.The statistical properties of the stimuli used to probe these neurons affect the properties of the surround as well. Because these areas are so highly interconnected, stimulation of one cell can affect the response properties of other cells, and therefore researchers have become increasingly aware of the choice of stimuli they use in these experiments. In addition to studies with simple stimuli (dots, bars, sinusoidal gratings), more recent studies have used more realistic stimuli (natural scenes) to study these effects. Stimuli that better represent natural scenes tend to induce higher levels of suppression, indicating this effect is tied closely to the properties of natural scenes such as textures and local context. Surround suppression is also modulated by attention. By training monkeys to attend to certain areas of their visual field, researchers have studied how directed attention can enhance the suppressive effects of stimuli surrounding the area of attention. Similar perceptual studies have been performed on human subjects as well. Surround suppression was formally discovered in the visual pathway, and noticed first by Hubel and Wiesel while mapping receptive fields. The earliest parts of the visual pathway: the retina, Lateral Geniculate Nucleus (LGN), and primary visual cortex (V1) are among the most well-studied. Surround suppression has been studied in later areas as well, including V2, V3, V4, and MT. Surround suppression has also been seen in sensory systems other than vision. One example in somatosensation is surround suppression in the barrel cortex of mice, in which bending one whisker can suppress the response of a neuron responding to a whisker nearby. It has even been seen in the frequency response properties of electoreception in electric fish.