First order visual pathways .1 Ganglion cell pathways

There is a clear difference in the way receptive cells connect to bipolar cells and then to ganglion cells within the retina and these will functionally determine differences between object recognition and object localisation.⁴ The fovea (200,000 cones/mm²) defines the central field and is very rich in midget ganglion cells.

Midget cells constitute the parvocellular pathway to the lateral geniculate nucleus.

This pathway is specialised for spatial acuity, colour vision and fine stereopsis.⁵ Parasol ganglion cells, on the other hand, receive afferent signals from many bipolar cells and are more present in the peripheral area of the macula and the rest of the retina. They constitute the magnocellular pathway to the lateral geniculate nucleus and specialise in motion detection.⁵ These two principle pathways, with a third minor one called the koniocellular pathway, constitute the axonal neural tract that will connect the ganglion cells to the lateral geniculate nucleus within the thalamus.

(Figure 1).

Figure 1: Visual pathways from the retina to the central nervous system.

Illustration by OpenStax College in Chapter 14: The Brain and the Cranial Nerves, Unit 3 Regulation, Integration, and Control, in Anatomy & Physiology; Rice University, Houston

PART  4  –Visual  processing,  ageing,  and  driving  

There are also fibres directly connecting the retina to the superior colliculus. These provide the direct input of a visuotopic map that can be used to mediate saccadic eye movement. Similarly, fibres from ganglion cells also project to each pretectal nucleus and provide input about light intensity to help mediate afferent signals to the Edinger-Westphal nucleus and to control pupillary size. An additional pathway connects ganglion photoreceptive cells directly to suprachiasmatic nucleus (SCN). These unmyelinated fibres regulate cell activity within the SCN and have a modulating effect on the pineal gland and its melatonin secreting cells, which drive circadian rhythms.⁶

4.2.2 Within the lateral geniculate nucleus

The lateral geniculate nucleus (LGN) plays a major role in controlling the signals that are transmitted to the primary visual cortex.⁷ The LGN is organised in six layers in which afferent axons from ganglion cells project, to following a retinotopic organisation. The magnocellular pathway projects to layer one (contralateral eye) and two (ipsilateral eye), whereas the parvocellular pathway projects to layers three to six (layers four and six for contralateral eye and three and five for ipsilateral eye). The second order neurones extend their axons from the LGN to the primary visual cortex and form the parietal and temporal radiation of the optic radiation.

The LGN receives modulating connections from the reticular nucleus (thalamus) and from the sixth layer of the primary visual cortex, to which it also sends both magnocellular and parvocellular projections (Figure 2). It therefore constitutes a

“bottleneck to information flow”⁴ and interplays with the reticular nucleus to release gatekeeping and let through new relevant information.⁸

Figure 2: Thalamic modulation of visual ganglion cell projections to primary visual cortex during their relay in the lateral geniculate nucleus. Image from Sherman (2007).⁹

The primary role of the LGN is, therefore, to filter relevant information that will be transmitted to the primary visual cortex. The distinction of visual information from the midget ganglion cells and the parasol ganglion cells (more specialised in detecting motion) are maintained. Parvocellular cells from layers 3-6 in the lateral geniculate nucleus send projections to layer 4Cα, whereas magnocellular cells in layer one and two send their projections to layer 4Cß. Visual processing that occurs in the primary

visual cortex (V1) is highly dependent upon higher order processing units within the rest of the cortex and their multiple connections to underlying structures.

4.2.3 The pulvinar

The pulvinar plays a key role in relaying information between cortical areas.⁹ The pulvinar receives descending cortical projections from both layers five and six of the visual cortex. It then projects neurones to most cortical regions and therefore serves as the transthalamic regulator of higher level cortical sensorial processing. Furthermore, the lateral pulvinar is capable of controlling and gating outflow from the primary visual cortex (V1) without affecting the signal from the lateral geniculate nucleus to the primary visual cortex.¹⁰

4.2.4 Feed-forward and feedback control of cortical visual processing

Visual processing has been thought to be organised hierarchically with increased complexity when moving up to higher levels of the anatomical hierarchy.¹ This is nevertheless a simplified conceptualisation of visual processing and there is much more to visual processing than just its anatomical hierarchical organisation.¹¹ Lateral interconnections and feedback can modulate visual processing in a non-linear way.

This occurs during re-entrant processing;¹² parallel pathways of information flow such as the cortico-thalamo-cortical pathways;^{9, 13} dynamic information processing;

and when integrating vision in a multisensory and behavioural context.¹¹

Figure 3: Feedforward (blue) and feedback (red) pathways during visual processing. Image by Gilbert & Li (2013).¹⁴AIP, anterior intraparietal area; FEF, frontal eye fields; IT, inferior temporal area; LGN, lateral geniculate nucleus; LIP, lateral intraparietal area; MIP, medial intraparietal area; MST, medial superior temporal area; MD, medial dorsal nucleus of the thalamus; MT, medial temporal area; PF, prefrontal cortex; PL, pulvinar; PMd, dorsal premotor area; PMv, ventral premotor area; SC, superior colliculus; TEO, tectum opticum;

VIP, ventral intraparietal area.

In each neural network, visual processing is, therefore, modulated by feed-forward and feedback signals from other networks (Figure 3). In other words, even if the anatomical visual pathway can be described as a hierarchical organisation, visual processing is to be mediated within each local network by other networks that interact one with another and this is before the signal has even reached the primary visual cortex (Figure 4).

PART  4  –Visual  processing,  ageing,  and  driving  

Thalamus

Sensory Cortex

Sensorimotor Motor

A. Classical hierarchical sensory processing

Thalamus

Sensory Cortex

Sensorimotor Motor

B. Linear and non-linear sensory processing

Higher order Higher order

First order

Motor output Motor output

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Figure 4: An alternative to the classic hierarchical sensory processing model. Modified from Sherman (2005).¹⁵