Visual Pathway

The visual processing system in the brain is part of the central nervous system and processes information coming from the eyes. The whole visual system is very complex and not fully understood, but it involves all processing from the capture of the light by the eyes to the response of motor behavior and memory association.

The visual information is usually processed through the visual cortex, which is the largest area in the human brain. The visual cortex is located in the rear part of the brain, above the cerebellum and both hemispheres of the brain contain a visual cortex. However, the left hemisphere is responsible for processing the right visual field and the right hemisphere the left visual field.

The visual cortex processes sensory information in a hierarchical way, and dif-ferent regions have neurons reacting to difdif-ferent visual concepts. The information first flows through the primary cortex, composed of the V1 region and go into the

2.2. Emotional Experience Perception

Figure 2.5: Illustration of the location of the visual cortex in the human brain in the rear part of the cerebrum, above the cerebellum. It is possible to see the regions from V1 to V5. Modified from [53] based on [109].

deeper V2, V2, V4 and V5 regions. The neurons in the primary cortex usually respond to different line segments and orientations, while neurons in V4, for ex-ample, react to complete objects or movement. This hierarchical processing allows the information to be shared through all these areas, and each of them reacts and processes different levels of abstraction. Figure 2.5 illustrates the visual cortex regions. All these regions are driven by feedforward connections, however, they are modulated by feedback and lateral interactions.

The primary cortex, or V1 area, is the most studied area in the brain. It is also the simplest and probably the earliest area of the visual cortex to develop, and it is highly specialized for processing of static objects and simple pattern recognition [185]. The neurons in V1 tend to have a strong response to a small set of stimuli, which happens because the V1 area has the smallest receptive field size in the visual cortex. Meaning that the neurons in the V1 area tend to react to small changes in orientation, spatial frequencies and colors [11]. The information encoded by the V1 neurons are basically edge detectors, representing the local contrast between different small structures and colors on the visual field. This region has straight-forward connections with the other regions, providing this fast and simple processing to deeper and more complex structures [4, 278]. Recent research [11] shows that feedback connections change also the properties of the V1 neurons over time. At first, the neurons in this region detect the small structures and information, but after this information is processed, feedback connections to the V1 neurons make them sensitive to the more global organization of the scene, such as macro disturbances and movements.

The V2 region is the second major area in the visual cortex and it receives strong feedforward connections from the V1 neurons. The neurons in V2 encode orientation, spatial frequency, and color, as the V1 area, but they have a larger receptive field. That means that the neurons in V2 identify small objects and complex patterns in multiple orientations and in different regions in the visual field [103]. These neurons are strongly modulated by orientation and binocular disparity and thus can identify background and foreground information [245]. Also the neurons in this region code a small attentional modulation, identifying macro focus regions, such as a person’s shape.

The neurons in the V3 region are generally associated with the processing of global motion [27]. They receive feedforward connections from the V2 and V1 areas and are known to cover the complete visual field [200]. Usually, they encode coherent motion of large patterns, showing an understanding of what the movement means. They are mostly associated with the perception of gestures and body movements [254].

The area known as V4 receives strong feedforward connections from V2 and weak connections from V1. These neurons usually encode space relations between different objects, orientation, and color. Different from V2, the neurons in V4 en-code mostly patterns with small complexity, like general shapes (circles, squares).

Some research [109, 252] states that V4 is responsible for dealing with color pro-cessing, especially spatial contrast defined by different colored objects, for example, background-foreground identification based on different colors. Also, the neurons in V4 are strongly modulated by attention mechanisms [212], which have a strong influence on the firing behavior of the neurons. This behavior illustrates how subcortical mechanisms influence the visual processing.

The V5 area is also known as the middle temporal region (MT) and plays a major role in the perception of motion, integration of local motion in the global view and connections with the motor area, mostly for eyes movement. The V5 neurons receive connections from the V1, V2, and V3 neurons, and although the strongest connections are coming from V1 neurons [24], studies show that visual information reaches the V5 area even before it reaches V1 neurons [75]. The neurons in this region encode speed and direction of movements in the whole visual field, integrating local movements into the whole scene.

Therefore, we can see that the visual cortex regions process different visual information: some of them relate to spatial relation between objects and some to movement. Based on that, Milner and Goodale [111] propose the two-streams hypothesis. This hypothesis states that the visual systems process information in two brain pathways: the ventral and the dorsal stream. They exhibit anatomical, neurophysiological and behavioral evidence that the ventral stream participates in the visual cognition process, determining information about what the person is visualizing. The dorsal stream, on the other hand, is involved in the recognition and processing of where the object is, related to space. The dorsal stream processes the spatial information of what the person is visualizing, for example, the distance of the object to the person. Regions V3 and V5 are directly associated with the dorsal stream, while regions V2 and V4 are placed in the ventral stream. The

2.2. Emotional Experience Perception

Figure 2.6: Illustration of the ventral and dorsal streams together with the visual cortex. Modified from [53] based on [109].

V1 neurons are usually connected to all of the other regions and serve as a first processing step for most of the visual cortex processing. Figure 2.6 illustrates the ventral and dorsal streams.

The ventral stream is directly associated with object and form recognition.

Some research names it the “what” stream. The neurons in this region increase their receptive fields size in the deeper regions, which increases the complexity of objects recognized in the visual field. Attention and memory have a large influence on the processing of the ventral stream, giving this region a strong role in the judgmental significance of the visual field. It was shown, for example, that the damages in the ventral stream cause the inability of a person to recognize facial expressions and identifying emotional experiences [110].

The neurons in the dorsal stream region are connected directly to the V1 neu-rons and are known to be involved in the guidance of action and recognition of where some objects are in space. This explains the “where stream” name which is often given to the dorsal stream. The neurons in the ventral stream are di-rectly connected with the motor system and have interconnections with the ventral stream. The neurons in this region encode two distinctive things: a detailed spa-tial map of the visual field and the detecting of movements. They are responsible for the perception of body movements and gestures, identifying speed, orientation, and direction of these movements. Damages in the dorsal region can lead to an inability to perceive motion and description of complex scenes, focusing only on single objects.

Both ventral and dorsal streams contribute to the perception of emotional experiences. Focusing on the identification of emotion expressions, the processing

of facial movements and body postures gives us the capability to perceive what others are expressing. The visual cortex is a very complex region and yet not fully understood, but what we know so far about it enables us to understand better how visual emotion expressions are perceived.