Human visual performance has been shown to be better in the lower visual field than upper visual field [1-3], and in the horizontal meridian than vertical meridian in many classes of stimuli [4, 5]. However, the neural mechanisms of such lower visual field and horizontal meridian advantages in human visual processing remain incompletely understood [6-9]. Understanding the neural correlates of visual field responses may also be clinically important. This study aims to determine if processing of visual information in the brain is dependent on the location of stimuli in the visual field. We quantitatively examined brain activity in response to visual presentations to (1) the upper and lower hemifields and (2) the vertical and horizontal meridians using functional MRI (fMRI).

Fourteen healthy young participants (8 males and 6 females) aged 27.4±3.5 years old (age range=22-32 years old) were recruited and informed consent was obtained prior to participation. All fMRI data were collected on a 3 Tesla Siemens Allegra scanner. Blood-oxygenation-level-dependent (BOLD) images were collected using a single-shot echo planar imaging pulse sequence with the following parameters: TR=2 s, TE=26 ms, field of view=24x24 cm2, 96x96 imaging matrix, 2.5x2.5mm2 in-plane resolution and 28 contiguous 2.5 mm thick axial slices. Brain activity was assessed with 2 fMRI scans with both eyes open and focusing on a fixation point to establish the center of the visual field. Each scan was 5 minutes long consisting of 12 trials of alternating 12 seconds of rest and 12 seconds of visual stimulation. Each participant underwent two fMRI scans, where two visual stimulus patterns were compared during each scan by alternating the stimulus pattern between odd and even trials:

1. Upper (Fig. 1a) versus lower hemifield (Fig 1b)

2. Vertical (Fig. 1c) versus horizontal meridian (Fig 1d)

The stimuli were checkerboard patterns flashing at 8 Hz and were presented using the e-prime software (Psychology Software Tools, Inc., Sharpsburg, PA, USA). Mean BOLD % change was calculated in the activated brain regions for each participant using a z-score threshold of 2.3 after slice-timing correction and image realignment in the FSL software [11]. Paired Student’s t-tests were performed comparing the mean BOLD % change or brain activation volume of all participants between upper and lower hemifields, or between vertical and horizontal meridians with significance set at p<0.05.

Figure 1 shows the visual presentation patterns and representative brain activation in response to visual stimulation to the upper (Fig. 1a) and lower hemifields (Fig. 1b) and horizontal (Fig. 1c) and vertical meridians (Fig. 1d). Upper and lower hemifield presentations predominantly activated the ventral and dorsal regions of the occipital cortex, respectively, whereas vertical and horizontal meridian presentations activated different retinotopic regions of the occipital cortex. Both BOLD % change (Fig. 2a) and activation volume (Fig. 2b) were significantly greater for lower hemifield stimulation than upper hemifield stimulation. No apparent difference in BOLD % change was observed between vertical and horizontal meridian visual presentations (p=0.11) (Fig. 2a). However, visual presentation to horizontal meridian appeared to activate larger brain areas than visual presentation to vertical meridian (Fig. 2b).The present results agree with recent psychophysical and neurophysiological studies which showed asymmetries between upper and lower field visual information processing [1-3, 6-9]. Specifically, lower hemifield visual stimulation produced stronger brain responses and larger activated brain areas than upper hemifield, suggesting that the human brain may be more sensitive to the lower visual field than the upper visual field. The vertical and horizontal meridians are known to retinotopically map to the V1-V2 and V2-V3 borders, respectively, and may possess horizontal-vertical anisotropy in the human brain [5,6]. Although visual stimulation to the horizontal meridian activated larger brain areas than the vertical meridian, the lack of difference between strengths of brain responses to vertical and horizontal meridian visual stimulation suggests that the asymmetry in visual brain processing between upper and lower visual fields is more pronounced than that between vertical and horizontal meridians using current testing visual presentation paradigms. The current results may provide insight into functional deficits seen in individuals with visual field loss.