The cortical hierarchy

The cortical areas of the visual system are typically represented as a hierarchy of intercon-

nected areas (Lennie, 1998; Livingstone & Hubel, 1988) (see Figure 1.3). The LGN acts as

the relay centre for the visual pathway. The visual system is broadly divided into multiple

visual areas (Zeki, 1978). Each area is comprised of highly specialised receptive fields and

Receptive fields of a neuron

Receptive fields of a neuron represent a defined region in the visual field which elicits a

response when stimulated (Sincich & Horton, 2005). The receptive fields at the lowest level

areas, V1 and V2, represent simple features over a small area of the visual field and are then

passed to the higher cortical areas such as V3, V4 and V5. The receptive fields of these areas

are larger and pool the sensory information across several lower level receptive fields (Burr

& Thompson, 2011; Gilbert et al., 2001). The strength of response of the neuron depends on

how well the stimulus is matched to its receptive field. In early areas, receptive fields are

tuned to simple properties such as orientation and scale. Cells in these areas are also tuned to

properties such as direction of motion and colour. Receptive fields in higher areas tend to

be larger, more specialised, more broadly tuned to dimensions such as scale, and respond to

more complex features.

The primary visual cortex

The first cortical visual processing level is the primary visual cortex (V1). It is the largest of

all visual areas of the cerebral cortex in the macaque (Felleman & Van Essen, 1991), and is

comprised of six sub-layers (see Figure 1.3). V1 is retinotopically organised and incoming

projections from the LGN are organised by the eye of origin. V1 extracts information of ocular

origin, orientation tuning and visual space and organises these in columnar functional maps

(Callaway & Nassi, 2009). The receptive fields in V1 are small and only respond to a very

restricted area of the visual field (Furlan & Smith, 2016; Lamme, 2003; Simoncelli & Heeger,

fields to build elongated receptive fields. The receptive fields resemble Gabor functions

with odd-symmetric or even-symmetric arrangements of on- and off-regions (Figure 1.4(b)).

These cells represent simple visual dimensions, such as the position, orientation and scale of

local image features (Ahissar & Hochstein, 1997). The specificity of cells in primary visual

cortex was identified using micro electrodes to record the activity of cells in cats (Hubel

& Wiesel, 1959). The position and orientation of the stimulus was crucial in obtaining a

strong reading from a specific neuron. Physiological evidence from a range of animals has

shown that neurons in the primary visual cortex are tuned to orientation (DeValois et al.,

1982; Hubel & Wiesel, 1962), spatial frequency (DeValois et al., 1982; Movshon et al., 1978)

and direction (Livingstone & Hubel, 1988; Movshon & Newsome, 1996). Consequently,

this has also been supported by electrophysiological findings, from the study of area V1 in

monkeys (Gilbert et al., 2001) and functional magnetic resource imaging (fMRI) in humans

(Seitz et al., 2005).

Figure. 1.4 (a) Simple cells in V1 sum the inputs from an array of LGN receptive fields that resemble an elongated bar with an on-centre and off-surround. (b) These receptive fields can be modelled using odd-symmetric and even-symmetric Gabor functions fields

Area V2 is the second largest visual area in the macaque, and receives two thirds of

its input from V1 (Sincich et al., 2003). V2 plays an important role in the processing of

orientation, colour, depth (Sincich & Horton, 2005) and motion (Hu et al., 2018). Recently,

area V2 has also been shown to contribute to binocular depth perception (Hu et al., 2018).

Psychophysical experiments are also used to investigate the behavioural responses of

receptive fields. Local visual tasks are defined as those that can be performed reliably based

on the type of information which individual cells in early cortical areas such as V1 encode.

These include the detection of stimuli and the discrimination of their orientation, position

and direction of motion.

Extrastriate Visual Areas

The higher levels in the visual pathway, such as V3, V4 and V5, have neurons with much

larger receptive fields. The tuning properties of these neurons are more complex, and their

responses are less dependent on the location and retinal size of stimuli, the viewpoint of

the observer and the prevailing lighting conditions (Felleman & Van Essen, 1987; Furlan

& Smith, 2016; Hubel & Wiesel, 1965; Mikami et al., 1986; Movshon et al., 1978; Sillito

et al., 2006; Zeki, 1974). The receptive fields of neurons in higher cortical areas integrate

information to represent global stimulus properties (Amano et al., 2009; Bex & Dakin,

2002; Burr & Thompson, 2011; Gilbert et al., 2001; Nishida, 2011) across a collection

of V1 receptive fields. At each step up the hierarchy the receptive cells get progressively

more complex in their tuning. Higher cortical areas, beyond V4, begin to display category

preferentially to faces (Blake & Wilson, 2011). Furthermore, object-specific cells have been

identified in the lateral occipital cortex (Grill-Spector et al., 2001).

In psychophysical and physiological studies, global processes are investigated using

stimuli or tasks that can only be resolved through integration and segregation of coherent

or conflicting information (Burr & Thompson, 2011; Garcia et al., 2013; Nishida, 2011). A

canonical example of this is the global motion stimuli introduced by Williams and Sekuler

(1984). These stimuli contain dots moving in multiple directions, and the observer’s task is to

determine the direction of motion of the stimulus as a whole. Information in local regions of

the image (and thus also the responses of V1 neurons) will reflect the direction of motion of

individual dots, and the task can only reliably be completed by integrating information about

the directions of all the dots across space. Cortical cells higher in the processing hierarchy

are involved in the perception of global aspects of a percept and generalise across individual

features such as spatial frequency, speed, motion and orientation. Receptive cells in the

global processing levels pool sensory information from a selection of lower level receptive

cells (see Figure 1.5).