How We Scan the Visual Scene Using Top-Down Brain Control

We see details in only a small portion of the world in front of us, one point at a time, guided by and processed in regions of the brain detected in new research described by Christian Kiefer and his colleagues in Frontiers in Neuroscience in February 2022.

How the eyes move to focus on important regions of the visual scene has been known for years but not on a conscious level. The eyes pause, or fixate, on an important point, like one corner of someone’s mouth, for less than 1/4 second on average, and then quickly jump to another point, such as the nose. The visual system puts such still pictures together somewhat like the way a digital camera can assemble a panoramic view from a series of still photos. But how does the brain aim and process these photo-like images?

A path of an observer’s eyes scanning a face, superimposed over that image. Dots show pauses (fixations); lines show eye movements.

Source: WIKIMEDIA COMMONS/SPOOSPA

That’s a question behind the research of Kiefer and colleagues when they combined magnetoencephalography (MEG) to scan the brains along with eye tracking to follow the gaze of in 31 volunteers presented with pictures of everyday objects like flowers, animals, and houses. This complex method measures the magnetic fields generated by electric currents in the brain. It provides an accurate resolution of the timing of neuronal activity. This noninvasive test uses recording from sensors outside the scalp.

Their results showed that activation of several regions of the cerebral cortex are correlated with pauses or fixations of the eyes as they scanned pictures both looking normally or searching for particular items. The junction between the temporal and parietal lobes, or temporoparietal junction, in the right cerebral hemisphere stood out as a central part of a network activated during the brief fixations of the eye while it scanned a visual scene during a guided search for objects. In addition, greater activity of that region was associated with recognizing those objects more quickly.

Temporoparietal junction on MRI

Source: M. Graziano/Creative Commons CC BY 3.0

Top-Down Control

Visual attention can be directed by a top-down system, governed by specific brain regions. Or it can be due to visual features of objects in the scene, such as their size, movement, or color, described as a bottom-up system. The research of Kiefer and his colleagues focused on the importance of top-down control.

When we consider how the eye works as an optical system, the central part of the retina, responsible for central vision, is most important for seeing details, or visual acuity. That’s because so many receptors--the cone cells--and the ganglion cells they connect to are clustered in such a small space. You can compare it to high-definition video, with many small pixels per inch. This detailed central vision can detect a few letters at a time in a word using 12-point font at a normal reading distance. Another estimate is that it encompasses about the same area as your fingernail viewed from an outstretched arm. Signals from central vision activate a disproportionate amount of space in the visual cortex, which receives information from the initial levels of the visual system and is necessary for visual perception.

Surrounding this central vision area is a larger area of the retina, and a corresponding part of the visual field, producing peripheral vision, which is much less detailed but is important for detecting off-center objects that might be important to attend to next.

The Temporoparietal Junction

As we scan the words on a screen or the objects in a picture, our eyes move from point to point with brief pauses or fixations during which the visual system takes in information and proceeds onward to cover other parts of the scene. How signals from the eye proceed from the retina and optic nerve to the visual cortex has been studied extensively, but how the resulting neural activity involves higher levels of the brain is an active area of research. The research described here looked at how such brain regions control attention to and processing of images in a top-down system. Although several brain regions were involved, one stood out. According to the authors,

Taken as a whole, these findings support the hypothesis that the temporoparietal junction...is a central part in the connectivity networks during guided visual searching and that it is involved in object recognition and the guidance of eye movements.

The research described here looked at how such brain regions control attention to and processing of images in a top-down system. Similar systems might be involved when we pay attention via other senses, such as our hearing when we talk with another person while ignoring other sounds in a restaurant.

Copyright (c) 2022 by Robert A. Lavine