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light intensity increases, the photoreceptors are decoupled so as to obtain maximum
resolution (DeValois & DeValois, 1988).
Bipolar cells: Second retinal level
There are two types of bipolar cells. The first category responds in the same way as
the photoreceptors to which they are connected (non-inverting cells) while the rest
invert the direction of polarisation (inverting cells). A bipolar cell is connected to the
photoreceptors either directly or via horizontal cells.
Ganglion cells: Third retinal level
The ganglion cells are divided into three categories depending on their functional prop-
erties. The difference between the two main categories, X and Y cells, is their property
of summation of the receptive field. The X cells give responses that are very close to
that of the bipolar cells. Their receptive field has activator and inhibitor regions. Their
response is linear. At the temporal level, their response is tonic or steady as it starts
with the appearance of the stimulation, lasts till the stimulation is applied and ends
after extinction. On the other hand, the temporal response of Y cells is in phases or
transitory. They do not respond in a continuous fashion to the prolonged applica-
tion of a given stimulation, but rather have a significant activity at the appearance or
extinction of the stimulation.
3.2.1.5 The concept of spatial frequency
A sound is characterised by its frequency, i.e. the speed of its oscillations. Similarly,
for a visual stimulus, we can define a measurement of sharpness of its forms. This is
the concept of spatial frequency. It can be defined as follows: Spatial frequency is the
number of cycles of a periodic oscillation of light or colour in one degree of visual
angle. Its value is given in terms of cycles per degree (cpd), as shown in Figure 3.6.
Though we are giving just a passing description of spatial frequency; it can be
defined in a strict mathematic framework (Perrin, 1998). In 1968, Campbell and
Robson had suggested that the visual system can be made of quasi-linear and indepen-
dent groups of band-pass filters, each centred quite closely on a spatial frequency band,
Figure 3.6 Definition of spatial frequency. In this example, it is 4 cpd
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