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The role of the MT/V5 brain region in the visual perception of motion
Disclaimer requested by my University: Students are reminded copying this essay is plagiarism. Suggested citation at end of essay. Thank you.
Neuroscientists’ discovery that different attributes of the visual system are processed by distinct regions within cerebral cortex can be traced back more than 150 years (Zeki et al., 1991).
Evidence for which functional specialisms are responsible for specific visual system attributes (i.e. motion, colour, form, depth, etc) is more recent but abundant.
The V5 brain region, or the middle temporal (MT) area, is the specialist function of visual motion perception in monkey and man (Beckers and Homberg, 1992, citing Newsome et al., 1985; Baker et al., 1991). Since this discovery, the V5/MT region has become one of the most meticulously studied brain regions (Zeki, 2015).
The Zeki et al (1991) study
Zeki et al (1991) devised a series of experiments to identify the separate brain centres responsible for motion and colour analysis within the human visual cortex. The two-experiment comparison study monitored regional cerebral blood flow (rCBF) changes via positron emission tomography (PET) scans.
PET scans identified an accurate topographical picture of specific cortical regions activated by the two independent variables (IV), motion and colour stimuli (Berger, 2003; Walsh et al, 1998).
The study demonstrated the relationship between regions of the visual system in relation to the two IVs: specifically, the striate cortex, V1, and the contiguous visual area, V2, with the V5/MT region under motion stimuli and with the V4 visual area, under colour stimuli (Zeki et al., 1991).
The dependent variables (DV) were six sequential rCBF measurements, conducted in a balanced order (ABCCBA) to control for time and habituation effects (Zeki et al., 1991).
The hypotheses were: motion stimuli would stimulate rCBFs changes within the V5/MT region; colour stimuli would stimulate rCBFs changes within the V4 visual area, and a positive relationship exists between the V1/V2 regions and V4 for colour processing, and V1/V2 regional and the V5 region for motion processing (Zeki et al., 1991).
The stimuli comprised: two versions of computer-generated stationary and random moving patterns for the moving-related experiments; and Land’s Mondrain display for the colour-related experiments.
The chosen stimuli were selected its proven utility in similar studies, including utility in accurate selective activation of targeted visual regions.
The effect of naturally occurring global CBF changes was identified and mitigated a potential confounding variable by statistical significance measurement via an analysis of covariance (ANCOVA) analysis.
PET scans were analysed by statistical parametric maps (SPMs), a well-regarded method for assessment of brain activity differences (Rowe, 2005).
The results supported the hypothesis. The two experiments showed statistically significant changes in mean rCBF variations located in the SPMs.
The results provided compelling empirical support to the theory that functional specialisation is integral to the human visual cortex operation.
Further, the study provided indirect evidence on anatomical connections between the visual areas responsible for motion and colour processing. However, the experiment design contained challenges.
PET imaged required radioactive tracer injections which limits use among healthy volunteers, repeatability and is low in signal-to-noise ratio (SNR) and provides limited temporal and spatial resolution (Chen et al., 2008; Zeki et al., 1991).
The utilisation of functional MRI scans (i.e. 3D images of complete brain activity) would have overcome the latter restriction (Pinel and Barnes, 2018).
The experiment design was limited by the difficulty in isolating areas of the visual system without the stimulation of other regions. To mitigate, indirect insights were sourced by contrasting scans with stimuli identical in all respects save the one of interest to.
The Treue and Maunsell study
Treue and Maunsell (1996) studied how attention influences neural activity in visual information-processing by conducting neuroimaging experiments on well-trained macaque monkeys. The experiments compared individual neural response activity differences to an identical visual stimulus under multiple attention scenarios.
Two monkeys were trained to hold their gaze on an initial fixation point on a dark computer monitor, which was later complicated by additional distractor stimulus. Response histograms recorded discrete neural activity relative to whether attention was directed inside or outside monkeys’ receptive field, as well as the dots’ direction of travel (Treue and Maunsell, 1996).
Only correctly completed trials were analysed. Monkeys’ eyes had to remain focused upon the initial fixation point to ensure neural activity monitoring within the MT and the medial superior temporal (MST) area was exclusively identifying behavioural state.
In the first experiment, stimuli were separately placed inside and outside the receptive field to monitor response variations when monkeys shifted attention towards, and away from, the stimulus. During the experiments, identical visual stimulation was maintained.
In the second experiment, a second dot was placed inside the receptive field moving parallel and in the opposite direction to the other dot (Treue and Maunsell, 1996; Martınez-Trujillo and Treue, 2002).
The neural response was strongest when monkeys were directed to focus on one of the dots within the receptive field and when the dots were travelling upwards, the cell’s preferred direction.
Movement of travel was a more powerful driver of neural activity than the potency of visual stimuli in the distractor dot. Conversely, neural response within the MT and MST regions was lower when attention was directed to the other visual hemifield.
The results demonstrated a significantly stronger neural response when monkeys paid attention to stimulus inside their receptive field which implied a greater influence of attention on motion processing within the MT and MTS than previously appreciated.
The use of differential attention in experiment 2 revealed robust attentional effects in MT, which hitherto had been unsupported in PET studies of human parietal cortex (Treue and Maunsell, 1996).
In addition, the demonstrated attentional effects in MT implied neural activity was substantially influenced by behavioural state and that visual information-processing is not simply a function of visual sensory inputs.
The experimental design utilised single-unit extracellular recordings which are invasive, requiring electrodes to be implanted in the brain, which is difficult and time-consuming which explains limited human studies to date (Carlson, 2014). The SNR is also considered weak. Thus, the repeatability of this experiment with humans is unlikely.
These studies, and others, demonstrate that functional specialisation is integral to visual cortex operation with information-processing more complex than simply sensory inputs. However, human studies via neuroimaging experiments are limited by inherent invasiveness.
This essay was submitted as part of my MSc in Psychology at Leeds Beckett University in January 2020. Students are reminded copying this essay is plagiarism.
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