At a cortical level, the upper and lower visual hemifields are respectively represented on the lower and upper cortical sheets of the occipital lobe (see Figure 4). The division in area V1 separates the two vertical hemifields within each hemisphere. Previc (1990) suggested that anatomical segregation promoted functional specialization for the different types of visual processing and argued that perceptual capacities of the visual field have been shaped by environmental constraints. Based on an evolutionary perspective, he proposed that the lower visual field became specialized for near vision because of its involvement in reaching and other manipulations performed in peripersonal space, whereas the upper visual field became specialized in visual search and object recognition toward extrapersonal space. When fixing a target object (e.g. while picking fruit), reaching and grasping occurs in the lower visual field, while visual search for and recognition of objects occurs in the upper visual field. Each hemifield adapted its type of visual processing based on the task that it usually performed. In the lower visual field, the principal task is the perception and manipulation of closer objects which require visuomotor coordination and spatial perceptual abilities. As objects are manipulated in this hemifield they could appear blurred and in motion, so Previc also proposed that the lower hemifield specialized in sensory processing of low spatial and high temporal frequencies (global processing). In contrast, in the upper hemifield, the principal task is to search for and recognize distant objects. Since object recognition requires the discrimination of fine details, the upper visual field specialized in high-level perceptual processing of high spatial frequencies (local processing).
Furthermore, Previc proposed that vertical asymmetries can be related to the two functionally and anatomically separate processing streams for visual processing, the ventral and the dorsal stream. Indeed, at an anatomical level, the lower cortical sheets (upper visual field) project more into the ventral stream of the temporal lobe, whereas the upper cortical sheets (lower visual field) project more into the dorsal stream in the parietal cortex (Fecteau, Enns, &
Kingstone, 2000). The ventral stream (also known as the “what” pathway) is involved in object identification and recognition, whereas the dorsal stream (also known as the “where” pathway) mainly processes visuospatial information, such as motion, distance and location (Goodale &
Milner, 1992) (see Figure 4), which is consistent with the specialization of hemifield mentioned above. This dichotomy between upper and lower visual fields seems to begin before visual processing.
At the retina level, there is a higher concentration of receptors and ganglion cells in the superior than in the inferior hemiretina at eccentricities exceeding 6° (Croner & Kaplan, 1995;
Curcio & Allen, 1990). Recent findings concerning monkeys have shown that representation of the visual space in the superior colliculus was not symmetrical (Hafed & Chen, 2016) as it was commonly assumed (Ottes, Vangisbergen, & Eggermont, 1986). The superior colliculus is involved in visual-motor processing, target selection and attention. This layered midbrain structure contains retinotopic maps of the visual field in superficial layers and spatially registered eye movement maps in deeper layers (Sparks & Nelson, 1987). Hafed and Chen (2016) have found that receptive fields in the upper superior colliculus were smaller with a higher spatial resolution compared to lower superior colliculi. In a similar vein, Mayo, DiTomasso, Sommer, and Smith (2015) reported that ~70% of response fields in the frontal eye field were upper visual field neurons. The frontal eye field is involved in eye movement exploration; the over representation of upper visual field neurons both in the frontal eye field and superior colliculus support the assumption of upper visual field specialization in visual exploration and object recognition. On the contrary, the higher concentration of receptors and ganglion cells far from the fovea in the superior retina and the large receptive fields in the superior colliculus support the assumption that the lower hemifield is specialized in motion/sensory processing.
Figure 4. Illustration of vertical visual asymmetries and processing streams. The superior hemifield is represented
in the ventral part of the occipital cortex whereas the inferior hemifield is represented in the dorsal part of the occipital cortex. The dorsal stream (“where” pathway) project into the parietal lobe whereas the ventral stream (“what” pathway) project into the temporal lobe.
Attentional Asymmetries
However, the evaluation of Previc’s hypothesis is mixed, particularly with respect to attentional asymmetries. For instance, He, Cavanagh, and Intriligator (1996) claimed that attentional resolution was better in the lower visual field by showing lower visual field advantages in difficult search tasks, but no differences in basic search tasks. In contrast, Carrasco, Talgar, and Cameron (2001) showed that attention, as measured by the effects of exogenous cueing, was similar in the upper and lower visual fields, whereas perceptual performance was better in the lower visual field. Meanwhile, Rezec and Dobkins (2004) showed that discrimination of basic visual features was better in the lower visual field when search involved the complete visual field, suggesting an attentional bias to the lower visual field, whereas discrimination at cued locations in the upper or lower visual field were equal, suggesting that perceptual performance was about equal. Overall, vertical asymmetries, just like horizontal asymmetries (as mentioned above), seem to rely on task demands and selective attention. Competitive mechanisms between the two streams have been revealed by varying task demands (Jokisch & Jensen, 2007). Additionally, Milner and Goodale (1995) introduced the distinction between the dorsal and ventral streams in terms of the behavioral goal. They
proposed that, depending on the purpose, the same attribute (e.g. orientation) can be processed in the ventral stream when judgments have to be made and reported, or in the dorsal stream if orientation is used to guide an action.
Despite the mixed results for attentional asymmetries, more recent research supports Previc’s hypothesis that sensory processing is improved in the lower visual field, whereas the upper visual field is specialized in conscious perception and object recognition.