Treisman’s ideas suggest that image features like colour and motion are analysed separately at an early stage of visual processing. As we shall see, this is consistent with evidence from anatomical and physiological studies of the visual system, and studies of humans with certain kinds of brain damage. Up to now we have discussed visual neurons as feature detectors, [feature detector a mechanism sensitive to only one aspect of a stimulus, such as red (for the colour dimension) or leftwards (for direction of motion) and unaffected by the presence or value of any other dimension of the stimulus] responding best to certain aspects of the retinal image, such as the orientation or direction of movement of an edge. But recent studies suggest that, rather than forming part of a single homogeneous visual system, the feature detectors are embedded in several different sub-systems, in which information is processed separately, at least to some extent. Magno and parvo cells The rods and cones in the retina function in d m and bright light, respectively. The cones are of three types, which are selective to different, if overlapping, ranges of light wavelength. The information from the cones is reorganized in the retina to give green–red and blue–yellow opponent channels. There is, in addition, a group of large retinal cells alongside the smaller colour-opponent cells. These large cells respond to the difference between the luminances (of any wavelength) in their centre and surrounding regions. They could be described as black–white opponent channels. The large cells are known as the magno or M cells, [magno (M) cell a large cell in the visual system (particularly, the retina and lateral geniculate nucleus) that responds particularly well to rapid and transient visual stimulation] contrasting with the colour-sensitive parvo or P cells (the names are taken from the Latin words for ‘large’ and ‘small’ respectively). [parvo (P) cell a small cell in the visual system (particularly, the retina and lateral geniculate nucleus) that responds particularly well to slow, sustained and coloured stimuli] The M cells differ from the P cells not only in their lack of colour selectivity and their larger receptive field sizes, but in being more sensitive to movement and to black– white contrast. M and P cells both receive inputs from both cones and rods, but M cells do not distinguish between the three cone types and so respond positively to light of any wavelength, whether dim or bright. The motion properties of M cells are exceptionally important. They respond to higher frequencies of temporal flicker and higher velocities of motion in the image than P cells do. Indeed M cells signal transients generally, while the P channels deal with sustained and slowly changing stimulus conditions. For example, a dim spot of white light switched on or off seems to appear or disappear suddenly, whereas a dim spot of coloured light seems to fade in or out gradually (Schwartz & Loop, 1983). This supports the hypothesis that different flicker/motion sensations ccompany activation of M and P channels. Most famously, Livingstone and Hubel (1987) ascribed colour sensations to P cell activity, motion and distance (depth) to M cell activity, and spatial pattern analysis to a combination of both. This tripartite scheme was based on a reorganization of the retinal information that subsequently occurs in the cerebral cortex. The optic nerve carries signals to a pair of nuclei near the centre of the brain called the LGN (lateral geniculate nuclei), and from there the signals are sent on to the primary visual cortices (area V1) at the back of the brain .There are perhaps 100 million cells in each of the left and right areas V1, so there is plenty of machinery available to elaborate on the coded messages received from the retina.
[David Hubel’s (1926– ) discovery, with Torsten Wiesel, of the orientation tuning of cells in the primary visual cortex initiated an entire industry investigating how the visual scene can be encoded as a set of straight-line segments. Their theory also became a cornerstone for serial processing models of visual perception. Later, though, with Margaret Livingstone, he supported the theory that visual features are processed in parallel streams stemming from magno and parvo cells in the retina.]
[David Hubel’s (1926– ) discovery, with Torsten Wiesel, of the orientation tuning of cells in the primary visual cortex initiated an entire industry investigating how the visual scene can be encoded as a set of straight-line segments. Their theory also became a cornerstone for serial processing models of visual perception. Later, though, with Margaret Livingstone, he supported the theory that visual features are processed in parallel streams stemming from magno and parvo cells in the retina.]
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