The Neural Basis of Visual Field Asymmetry in Human Visual System by Functional MRI
Caitlin O'Connell1, Leon Ho2,3, Matthew Murphy2, Yolandi van der Merwe1,2, Ian Conner1,2, Gadi Wollstein1,2, Joel Schuman1,2, Rakie Cham1, and Kevin Chan1,2

1Department of Bioengineering, University of Pittsburgh, Pittsburgh, PA, United States, 2UPMC Eye Center, Eye and Ear Institute, Ophthalmology and Visual Science Research Center, Department of Ophthalmology, University of Pittsburgh, Pittsburgh, PA, United States, 3Department of Electrical and Electronic Engineering, The University of Hong Kong, Pokfulam, Hong Kong


Human visual performance has been observed to exhibit superiority in the lower visual field and horizontal meridian compared to the upper visual field and vertical meridian, respectively, in response to many classes of stimuli, but the underlying neural mechanisms remain unclear. This study determines if processing of visual information is dependent on the location of stimuli in the visual field using functional MRI. The results show stronger brain responses and larger activation volumes upon flickering visual stimulation to the lower hemifield compared to upper hemifield, while only the activation size differed between visual presentations to the horizontal and vertical meridians.


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).

Discussion & Conclusion

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.


This work was supported by the National Institutes of Health Contracts P30-EY008098, T32-EY017271, and UL1-TR000005 (Bethesda, Maryland); BrightFocus Foundation G2013077 (Clarksburg, Maryland); Alcon Research Institute Young Investigator Grant (Fort Worth, Texas); Eye and Ear Foundation (Pittsburgh, Pennsylvania); and Research to Prevent Blindness (New York, New York).


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Figure 1. Sample checkerboard flickering visual stimuli to 4 different regions in the visual fields (left) and the corresponding fMRI activation maps (right) represented by z-scores threshold at z = 2.3 at the ventral and dorsal levels of the occipital cortex from a representative participant.

Figure 2. (a) Average BOLD % change in activated brain regions and (b) average brain activation size comparing between visual stimulation to upper and lower hemifelds (left) and between visual stimulation to vertical and horizontal meridians (right). Error bars represent the standard deviation. (Paired t-tests: *p<0.05, **p<0.01, ***p<0.001.)

Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)