When other areas of the brain interpret the incoming sensory impulses the process of has occurred?

Bottom-up processing is an explanation for perceptions that start with an incoming stimulus and working upwards until a representation of the object is formed in our minds. This process suggests that our perceptual experience is based entirely on the sensory stimuli that we piece together using only data that is available from our senses.

In order to make sense of the world, we must take in energy from the environment and convert it to neural signals, a process known as sensation. It is in the next step of the process, known as perception, that our brains interpret these sensory signals.

How exactly do people process perceptual information from the world around them? There are two basic approaches to understanding how this sensation and perception takes place. One of these is known as bottom-up processing and the other is known as top-down processing.

Bottom-Up

• Data driven

• Focuses on incoming sensory data

• Takes place in real time

Top-Down

• Info is interpreted using contextual clues

• Uses previous experience and expectations

Bottom-up processing can be defined as sensory analysis that begins at the entry-level—with what our senses can detect. This form of processing begins with sensory data and goes up to the brain's integration of this sensory information. Information is carried in one direction starting with the retina and proceeding to the visual cortex.

This process suggests that processing begins with a perception of the stimuli and is fueled by basic mechanisms developed through evolution. Unlike top-down processing, bottom-up processing is purely data-driven and requires no previous knowledge or learning. Bottom-up processing takes place as it happens.

For example, if you see an image of an individual letter on your screen, your eyes transmit the information to your brain, and your brain puts all of this information together.

The theory of bottom-up processing was introduced by psychologist E. J. Gibson, who took a direct approach to the understanding of perception. Rather than being dependent upon learning and context, Gibson felt that perception was a “what you see is what you get” process. He argued that sensation and perception are the same things.

Because Gibson’s theory suggests that processing can be understood solely in terms of environmental stimuli, it is something referred to as the ecological theory of perception.

Bottom-up processing works like this:

1. We experience sensory information about the world around us, such as light levels from our environment.
2. These signals are brought to the retina. Transduction transforms these signals into electrical impulses that can then be transmitted.
3. Electrical impulses travel along visual pathways to the brain, where they enter the visual cortex and are processed to form our visual experience.

This approach to understanding perception is an example of reductionism. Rather than looking at perception more holistically, including how sensory information, visual processes, and expectations contribute to how we see the world, bottom-up processing breaks the process down into its most basic elements.

You can compare how bottom-up processing works to how top-down processing works by considering examples of how each process works. Imagine that you see a somewhat obscure shape. If you saw the shape on its own, using bottom-up processing, you might immediately perceive it as a capital letter B.

Now if someone were to place that image next to other context clues, such as next to the numbers 12 and 14, you might them perceive it as the number 13 rather than a capital B. In this case, you use top-down processing to interpret the visual information in light of surrounding visual cues.

While the two processes are often presented as competing theories, both play an important role in perception. The experience of visual illusions, for example, can illustrate how bottom-up and top-down processes influence how we experience the world.

You have probably seen a number of visual illusions where random ink blobs initially just look like ambiguous shapes, but after a moment begin to look like a face. If we used only bottom-up processing, these ink blobs would continue to look just like random shapes on paper.

However, because our brains are predisposed to perceive faces and because of top-down processes, we are likely to begin to see a human face in these ambiguous shapes.

Prosopagnosia, also known as face blindness, is a neurological disorder in which people are unable to recognize familiar faces, including their own. While other aspects of visual processing and cognitive functioning remain unaffected, people experience functional sensation but incomplete perception. Patients are able to perceive familiar faces, but not able to recognize them.

In this case, bottom-up processing remains functional, but a lack of top-down processing makes them unable to relate what they are seeing to stored knowledge. This demonstrates how important both processes are in shaping our perceptual experiences.

Bottom-up processing can be extremely useful for understanding certain elements of how perception occurs. However, research has also shown that other factors including expectation and motivation (elements of top-down processing) can have an impact on how we perceive things around us.

Vision is the sense we most depend on in our daily lives, and it is complex - despite the huge strides recently made in artificial intelligence and image processing, the way our brains process images is vastly superior. So how do we do it?

From the eye to the brain

The axons of ganglion cells exit the retina to form the optic nerve, which travels to two places: the thalamus (specifically, the lateral geniculate nucleus, or LGN) and the superior colliculus. The LGN is the main relay for visual information from the retina to reach the cortex. Despite this, the retina only makes up about 20% of all inputs to the LGN, with the rest coming from the brainstem and the cortex. So more than simply acting as a basic relay for visual input from retina to cortex, the LGN is actually the first part of our visual pathway that can be modified by mental states.

The superior colliculus helps us to control where our head and eyes move, and so determines where we direct our gaze. Saccades, the jumpy eye movements that you are using as you read this text, are also controlled by the superior colliculus. As with the LGN, the superior colliculus receives strong input from the cortex, which provides the dominant command as to where our gaze moves.

Cortical processing of visual input

From the thalamus, visual input travels to the visual cortex, located at the rear of our brains. The visual cortex is one of the most-studied parts of the mammalian brain, and it is here that the elementary building blocks of our vision – detection of contrast, colour and movement – are combined to produce our rich and complete visual perception.

Most researchers believe that visual processing in the cortex occurs through two distinct 'streams' of information. One stream, sometimes called the What Pathway (purple in the image below), is involved in recognising and identifying objects. The other stream, sometimes called the Where Pathway (green), concerns object movement and location, and so is important for visually guided behaviour.

Selket/Wikimedia

Building our visual world step by step

Our visual cortex is not uniform, and can be divided into a number of distinct subregions. These subregions are arranged hierarchically, with simple visual features represented in 'lower' areas and more complex features represented in 'higher' areas.

At the bottom of the hierarchy is the primary visual cortex, or V1. This is the part of visual cortex that receives input the thalamus. Neurons in V1 are sensitive to very basic visual signals, like the orientation of a bar or the direction in which a stimulus is moving. In humans and cats (but not rodents), neurons sensitive to the same orientation are located in columns that span the entire thickness of the cortex.

That is, all neurons within a column would respond to a horizontal (but not vertical or oblique) bar. In a neighbouring column, all neurons would respond to oblique but not horizontal or vertical bars (see image below). As well as this selectivity for orientation, neurons throughout most of V1 respond only to input from one of our two eyes. These neurons are also arranged in columns, although they are distinct from the orientation columns. This orderly arrangement of visual properties in the primary visual cortex was discovered by David Hubel and Torsten Wiesel in the 1960s, for which they were later awarded the Nobel Prize.

Orientation columns in primary visual cortex, as viewed from above. All neurons within a column respond preferentially to bars of a specific orientation, denoted here by colour. Crair et als/Wikimedia 

Moving up the visual hierarchy, neurons represent more complex visual features. For example, in V2, the next area up in the hierarchy, neurons respond to contours, textures, and the location of something in either the foreground or background.

Beyond V1 and V2, the pathways carrying What and Where information split into distinct brain regions. At the top of the What hierarchy is inferior temporal (IT) cortex, which represents complete objects – there is even a part of IT, called the fusiform face area, which specifically responds to faces. The top regions in the Where stream are involved in tasks like guiding eye movements (saccades) using working memory, and integrating our vision with our body position (e.g. as you reach for an object).

In summary, the visual cortex shows a clear hierarchical arrangement. In lower areas (those closest to incoming light, like V1), neurons respond to simple visual features. As the visual input works its way up the hierarchy, these simple features are combined to create more complex features, until at the top of the hierarchy, neurons can represent complete visual objects such as a face.

Visual processing isn’t all one way

This bottom-to-top processing of our visual world may seem the logical path, but it isn’t the whole story. Such a 'bottom-up' approach would be far too slow and laborious, but more importantly, it would render our visual world full of ambiguity and we would struggle to survive. Instead, our perception relies to a very large extent on our previous experience and other 'top-down' mechanisms such as attention. QBI Professors Jason Mattingley and Stephen Williams are both studying how attention can alter visual processing, using cognitive and cellular approaches, respectively.

As an example of top-down processing, consider the image below:

Wuhazet - Henryk Żychowski

Square A looks lighter, but is actually darker than square B. Clearly, our visual system is doing a terrible job at seeing reality. But that isn’t its purpose. Instead, our brains are trying to make sense of what they are seeing, rather than seeking the truth.

In the case of the above image, we automatically see – based on past experience – light and dark squares arranged in a checkerboard fashion, with a centrally lit portion and a shadow cast around the edges. With all of this information, we interpret A as a light square in shadow, and B as a brightly lit dark square. It isn’t reality, but it is the most likely explanation given all of our previous experience and the data at hand. This is how our visual system works, ultimately to help us understand the world and so promote our survival.

Here are the squares side by side: 

And to end, just because it’s fun, here’s a video with a great demonstration and explanation of how powerful top-down processing can be in visual perception.

Author: Alan Woodruff