Chapter 5: Higher-Level Visual Processing: Beyond V1

Introduction

In our exploration of sensation and perception, we've traced the visual processing pathway from the eye to V1 (striate cortex) and discussed the divergence of information into the dorsal and ventral pathways. These pathways are crucial for higher-level visual processing, and they were first extensively studied by neuroscientists Leslie Ungerleider and Mortimer Mishkin. In this chapter, we'll delve deeper into these pathways, their neural basis, and their functions.

Dorsal and Ventral Pathways: A Fishy Mnemonic

The "Two-Stream" hypothesis makes a distinction between "What" and "Where/How" processing. For a memorable mnemonic, think of a fish: if you were a fish and had a dorsal fin on your back and continued this pathway from your back to the back of your head and over the top of your head then you would be tracing the dorsal stream ("Where" or "How") which travels to the parietal lobe, whereas the ventral stream ("What") travels from the back of your head down to the temporal lobe.

Dorsal Pathway (The "How" Stream)

• The dorsal pathway extends from V1 to the parietal lobe.
• Often referred to as the "Where" or "How" stream, it primarily deals with spatial information and guiding actions in response to visual input.
• Think of it as the pathway responsible for determining "how" to interact with objects in the environment, such as reaching for a cup of coffee.

Ventral Pathway (The "What" Stream)

• The ventral pathway extends from V1 to the temporal lobe.
• It is famously known as the "What" stream, as it focuses on identifying and recognizing objects, their properties, and features.
• This pathway is akin to telling you "what" the object is, like identifying that the cup of coffee contains your favorite latte.

Neural Basis of Dorsal and Ventral Pathways

Leslie Ungerleider and Mortimer Mishkin's research played a pivotal role in understanding the neural basis of these pathways.

• Magnocellular Layers (LGN): These layers are associated with the dorsal pathway, processing motion and spatial information.

• Parvocellular Layers (LGN): These layers contribute to the ventral pathway, allowing for the identification of object features like shape, color, and texture.

Double Dissociation from Brain Damage

Ungerleider and Mishkin's groundbreaking research included lesion studies that revealed a double dissociation damage on visual processing.

Temporal Lobe Damage

• Damage to the temporal lobe leads to difficulties in object recognition.
• Patients may exhibit visual form agnosia, where they struggle to identify objects, even though they can describe them in terms of shape, color, and other features.
• Temporal lobe damage also affects the ability to copy objects accurately.

Parietal Lobe Damage

• Damage to the parietal lobe results in difficulties with spatial processing and interactions with objects.
• Patients may experience optic ataxia, where they can identify objects but struggle to grasp and interact with them accurately.
• This damage primarily affects the "where" and "how" aspects of visual processing.

Double Dissociation

Together, visual form agnosia and optic ataxia offer a compelling illustration of a double dissociation. In the first case, an individual exhibits the ability to perform cognitive task A but struggles with cognitive task B. Conversely, in the second case, another individual demonstrates competence in cognitive task B but encounters difficulties with cognitive task A. This pattern of complementary cognitive deficits signifies a double dissociation, suggesting that these two cognitive functions are underpinned by distinct neural substrates and are, crucially, separable from one another. Here the ability to recognize and interact with an object are dissociable.

Such a double dissociation not only underscores the specialized nature of neural regions responsible for these cognitive functions but also provides robust evidence that these functions rely on discrete brain regions that can be selectively impaired or preserved following brain damage.

Continuous Flash Suppression (CFS)

Now, let's explore a fascinating technique called continuous flash suppression (CFS), which can render visual information unconscious by suppressing the ventral pathway while keeping the dorsal pathway active.

Determining Eye Dominance

Before we can understand CFS, it's crucial to determine a person's eye dominance. It turns out, just like hand dominance people have a dominant eye. One effective method for determining eye dominance involves the following steps:

1. Extend your hands in front of you (so your elbows are straight), creating a triangular gap between the pointer finger and thumb of each hand.
2. While keeping both eyes open, focus on an object through the gap.
3. Close your left eye. If the object disappears, you are left-eye dominant. If it remains visible, you are right-eye dominant.
4. Repeat the process, this time closing your right eye. If the object disappears, you are right-eye dominant.

Binocular Rivalry and Continuous Flash Suppression

CFS capitalizes on binocular rivalry, a phenomenon where each eye receives different visual information, leading to perceptual alternations between the two images. This can be achieved, for example, by presenting an individual with a superimposed image of a house and face, each selectively visible through differently colored filters. Specifically, the image of the house is perceptible when viewed through a blue lens but remains concealed when observed through a red lens. Conversely, the image of the face becomes apparent when seen through the red lens but remains obscured under the blue lens.

When an individual wears specialized red/blue glasses, with the red lens covering one eye and the blue lens covering the other, each eye receives a distinct image input. What makes this phenomenon particularly captivating is that the individual's conscious experience fluctuates intermittently. At times, they become consciously aware of perceiving the face, while at other times, they shift their conscious awareness to the image of the house.

Continuous Flash Suppression: A Unique Version of Binocular Rivalry

1. A person wears red-blue 3D glasses with the red lens over their dominant eye and the blue lens over the non-dominant eye.
2. Randomly changing visual noise (static) is shown to the dominant eye.
3. In the non-dominant eye, a clear image, such as an object or scene, is presented.
4. Despite the clear image being shown to one eye, the viewer's conscious experience is dominated by the rapidly changing static in the dominant eye.
5. This effectively suppresses the person's awareness of the clear image presented to the non-dominant eye.

Priming Study: In 2008, Almeida, Mahon, Nakayama, and Caramazza conducted a priming study with CFS that revealed interesting findings. Participants were exposed to images under CFS, making it difficult to identify the images consciously. However, when asked to categorize a subsequent image as tools or animals, they exhibited priming effects only for tools (i.e., faster to respond to a tool that was preceded by a tool relative to a tool preceded by an animal even though the first object was not consciously seen). Since this finding did not occur for animals and since people have experience grasping tools but not animals (the how of visual processing), this suggests that the dorsal stream may be involved in this priming effect.

Shape Processing in Continuous Flash Suppression: Recent research has challenged the notion that priming in continuous flash suppression reflects “how” level processing rather than information about the visual look of an object. Some studies indicate that in this state, our brains might primarily process the shape of objects rather than their meaning. For instance, priming effects were observed even when tools (long thin objects) were preceded by unrelated objects like snakes as long as the shape was the same. This results suggests that the processing might focus on the shape of the objects rather than how we will interact with the object.

Modularity in the Brain

Modularity refers to the idea that certain behaviors or mental processes (e.g., face recognition) are served by specific brain regions.

PET Imaging and Modularity:

Positron emission tomography (PET) further supported the idea of modularity in the brain. In a task that required participants to determine which of two images was a 90- degree clockwise rotation of an initial image, brain activity showed clear distinctions. Dorsal stream activity was prominent in the "where" pathway when determining spatial relationships (which of two random dot patterns was a 90-degree rotation of an initial random dot pattern). Similarly, ventral stream activity was prominent in the "what" pathway, when recognizing faces (which of two faces was a 90-degree rotation of an initial face).

Specialized Brain Areas for Faces and Places:

Visual perception is a complex process that relies on the coordinated activity of various specialized brain areas, each dedicated to processing specific types of visual information. Two critical brain regions that have garnered significant attention in the realm of visual processing are the Fusiform Face Area (FFA) and the Parahippocampal Place Area (PPA). The research conducted by Nancy Kanwisher and her colleagues has shed light on the specialized functions of these areas, sparking debates about the extent of their specificity.

Fusiform Face Area (FFA):

One of the pioneering figures in the field of face processing research is Nancy Kanwisher, whose work has illuminated the role of the Fusiform Face Area (FFA). Located in the right temporal lobe, the FFA is renowned for its remarkable specialization in processing faces. Studies utilizing functional magnetic resonance imaging (fMRI) consistently show heightened activity in the FFA when individuals view faces. This area not only responds to facial features but also plays a crucial role in facial recognition and discrimination.

Kanwisher's research has provided compelling evidence for the specificity of the FFA. When subjects are exposed to images of faces, there is a distinct and robust increase in FFA activity. This distinctiveness implies that the FFA is finely tuned to facial processing and serves as a dedicated neural module for this purpose. Moreover, damage to the FFA can result in a condition known as prosopagnosia, characterized by the inability to recognize faces, even those of close family members or friends.

Parahippocampal Place Area (PPA):

The Parahippocampal Place Area (PPA), located in both the left and right temporal lobes, specializes in processing scenes, environments, and spatial layouts. PPA activity significantly increases when individuals view images of landscapes, buildings, or other place-related stimuli. This area plays a pivotal role in spatial navigation and our ability to recognize and navigate through different environments.

Research investigating the PPA has demonstrated its specialization for place processing. Studies utilizing fMRI consistently reveal heightened PPA activation in response to place-related visual stimuli. Moreover, individuals with damage to the PPA often experience difficulties in recognizing or navigating through spatial environments.

Debate Surrounding FFA and Its Generalization to Other Stimuli:

While the FFA's specialization for faces is well-established, there has been an ongoing debate about whether this area can process other types of visual information. Nancy Kanwisher's work primarily focused on faces, but the question arose: Can the FFA respond to complex visual stimuli other than faces?

Isabel Gauthier's research has provided significant insights into this debate. Her studies have examined whether individuals can become experts in recognizing non-face stimuli, such as greebles (novel objects), cars, or birds, and if so, whether the FFA plays a role in processing these stimuli. Gauthier's findings suggest that individuals who become experts in recognizing non-face stimuli exhibit FFA activation when exposed to these stimuli, demonstrating that the FFA may not be as strictly specialized as previously thought.

Conclusion

In this chapter, we have ventured beyond the primary visual cortex (V1) to explore the intricacies of higher-level visual processing. We discussed the two major pathways out of V1 (dorsal and ventral) and the concept of modularity. As we conclude this chapter, we are left with a profound appreciation for the complexity and versatility of the human visual system, which continues to be a source of inspiration and exploration for researchers seeking to unlock the secrets of perception, cognition, and consciousness.