Neural response during temporal - and spatial luminance contrast processing and its manifestation in the blood-oxygen-level-dependent-signal in striate and extra-striate cortex.

The primate visual system has been the prime site for investigating the relationship between stimulus property, neural response and blood-oxygen-level-dependent (BOLD)-signal; yet this relationship remains ill-understood. Electrophysiological studies have shown that the ability to visualise a neural response is determined by stimulus property and presentation paradigm. The neural response in the human visual cortex consists of a phasic response processing temporal and tonic response processing spatial luminance contrast. We investigated their influence on the BOLD signal from the visual cortex. To do so, we compared BOLD signal amplitude from BA17 and BA18 of 15 human volunteers to visual patterns varying the size of the active neural population and the discharge activity of this population. The BOLD signal amplitude in both areas reflected the discharge activity of the active neural population but not the size of the active neural population. For identical stimuli, BOLD signal amplitude in BA17 exceeded than that of BA18. This indicates that the BOLD signal reflects the tonic neural neuronal response during spatial luminance contrast processing. The difference in BOLD signal amplitude between BA17 and BA18 is accounted for by differences in neurophysiological and cytoarchitectonic differences between the two areas. Our findings offer an understanding of the relationship between stimulus property, neural response and the BOLD signal by considering the cytoarchitectonic, and neurophysiological make-up between different cortical areas and the influence of a phasic and tonic neural response on local deoxyhaemoglobin concentration. Conversely, differences in BOLD signal between brain structures and stimuli provide cues to the influence of different neurophysiological mechanisms on the neural response.