Sensitivity to the visual field origin of natural image patches in human low-level visual cortex

Asymmetries in the response to visual patterns in the upper and lower visual fields (above and below the centre of gaze) have been associated with ecological factors relating to the structure of typical visual environments. Here, we investigated whether the content of the upper and lower visual field representations in low-level regions of human visual cortex are specialised for visual patterns that arise from the upper and lower visual fields in natural images. We presented image patches, drawn from above or below the centre of gaze of an observer navigating a natural environment, to either the upper or lower visual fields of human participants (n = 7) while we used functional magnetic resonance imaging (fMRI) to measure the magnitude of evoked activity in the visual areas V1, V2, and V3. We found a significant interaction between the presentation location (upper or lower visual field) and the image patch source location (above or below fixation); the responses to lower visual field presentation were significantly greater for image patches sourced from below than above fixation, while the responses in the upper visual field were not significantly different for image patches sourced from above and below fixation. This finding demonstrates an association between the representation of the lower visual field in human visual cortex and the structure of the visual input that is likely to be encountered below the centre of gaze.

There are numerous reports of asymmetries in the visual system's representation of the upper and lower 2 visual fields (that is, above and below an imaginary horizontal line that passes through the centre of pattern would depend on its source in the visual field. 25 Here, we tested this hypothesis using functional magnetic resonance imaging (fMRI) of human 26 low-level visual cortex. We used image patches obtained from above and below the centre of gaze of 27 an observer freely-navigating an outdoor environment (Schumann et al., 2008), and presented them to 28 the upper or lower visual fields of observers. We predicted that the amplitude of the blood-oxygen-level 29 dependent (BOLD) signal in the low-level visual areas (V1, V2, and V3) would show an interaction 30 between the presentation location (upper or lower visual field) and source location (above or below 31 fixation); responses would be greater for image patches sourced from above than below fixation when 32 presented in the upper visual field, and vice-versa for the lower visual field.

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Participants 35 Seven participants, each with normal or corrected-to-normal vision, participated in the current study.   Each stimulus consisted of a set of apertures that were 4.5 • in diameter and centred at 5.37 • eccentricity 55 in the four visual field quadrants (see Figure 1). This aperture placement was designed to allow both 56 unambiguous presentation in either the upper or lower visual field (hence, away from the horizontal 57 meridian) and for the activation to be attributed to different visual areas with greater certainty (hence, away 58 from the vertical meridian). On a given trial, the apertures in either the upper or lower visual field displayed 59 natural image patches. The natural image patches were sourced from recordings by Schumann et al.

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(2008), which are aligned to the centre of the gaze of an observer freely-navigating an environment (that 61 is, the centre of the recorded image corresponded to the centre of the observer's gaze the null and image events was randomised, and the final 8 events were replicated and prepended to the 81 sequence to allow an initial period of each run to be removed while maintaining balanced trial counts. 82 Figure 1. Stimulus and experimental design. Each stimulus consisted of apertures in the visual field quadrants. On a given trial, the apertures in either the upper or lower visual field (presentation location) displayed a pair of natural image patches. These patches were sourced from either above or below the centre of gaze of an observer navigating a natural environment (source location). Each panel shows these four experimental conditions for an example natural image. The number shown at central fixation relates to a behavioural task; see Design for details.
visual fields. Overall, each run was 332 seconds in duration and participants completed 10 runs that were 88 collected in a single session.

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Participants were engaged in a challenging behavioural task during each run (see Mannion et al., 90 2013, for details). Briefly, participants were required to respond with a button press whenever one of two 91 target digits and polarities (black or white) was presented in a rapidly updated (3 Hz) stream at central 92 fixation (see Figure 1). This task had no direct relevance to the aims of this study, but was designed to 93 divert participant's attention away from the stimuli and hence to limit the effects of potentially unequal 94 attentional allocation to different stimulus conditions.    Table S1.  173 We find a significant interaction between the presentation and source locations (F 1,6 = 11.19, p = 0.016), 174 as is apparent in Figure 3. Because there was little evidence to indicate that this interaction depended 175 on the visual area (F 1.9,11.4 = 1.52, p = 0.260), we combined across the visual areas (V1, V2, and V3) 176 for subsequent analyses. Responses to images sourced from below fixation were higher than those from 177 above fixation when presented in the lower visual field, with the below and above sources eliciting   As discussed, the responses to image patches sourced from above and below fixation were unequal 196 when presented in the lower visual field-those sourced from above fixation evoked a reduced response.

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To gain insight into the characteristics of the above and below fixation image patches that may underpin  In addition to such analyses, based on distributions of pixel values, we also used a set of Gabor 222 filters (Daugman, 1985;Gabor, 1946;Marčelja, 1980) to simulate the response of spatial frequency and 223 orientation-selective neurons in low-level visual cortex. We find that higher levels of output for filters with  The current study also has further limitations that are suggestive of avenues for future research. First, 250 we are unable to generalise the results beyond the particular set of images that were used. Although 251 the images were chosen from an environment that is reasonably ecologically representative, in that it compliant with the instructions to direct their gaze to the central fixation task throughout the experiment.

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CONCLUSION 262 We found that the response in human low-level visual cortex to natural image patches presented in the 263 lower visual field depended on whether the patches were sourced from above or below the gaze of an 264 observer navigating a natural environment-patches sourced from below fixation evoked significantly 265 greater activity than patches sourced from above fixation, when presented in the lower visual field. This Reviewing Manuscript