In the retina of the human eye, there are two kinds of receptors: night-time vision and day-time vision. Specifically, cone-shaped receptors (cones) and rod-shaped receptors (rods) facilitate the two types of visions respectively. Photopic vision is mediated by the cones, which provides all the colours that one sees during the day, in bright light. On the other hand, scotopic vision is mediated by highly sensitive rods that allow one to see at night lacking colour perception. For photopic and scotopic vision, there are different spectral sensitivity curves: a photopic spectral sensitivity curve and a scotopic one which are determined by either shining a bright light into the test subject’s fovea or shown in their peripheral vision. Each participant is then asked to assess the brightness.
Spectral sensitivity is defined as how efficiently our eyes detect wavelengths of light, each curve peaks at a different point. The photopic spectral sensitivity curve is most sensitive in yellow light (a wavelength of around 560nm). Conversely, the scotopic sensitivity curve is most sensitive is blue light (a wavelength of around 500nm). This displays that under each condition, the converse would be more intense. This brings to light the Purkinje phenomenon, defined as “In intense light, red and yellow wavelengths look brighter than blue or green wavelengths of equal intensity; in dim light, blue and green wavelengths look brighter than red and yellow wavelengths of equal intensity.” (Barnes, Pinel 2017) Jan Purkinje first observed and analyzed this shift in vision when he realized that at night his blue flowers appeared bright than his red and yellow ones, however, during the day, the opposite effect. This shift in relative brightness, with reference to the photopic and scotopic spectral sensitivity curves, displays that our eyes are less efficient in perceiving colour during the night time than during the day.
Our eyes adapt to the dark, allowing us to see during the night time. Cones are only able to optimally function in brightly lit environments, whereas rods function optimally in dimly lit environments. Because cones are only found at the fovea, they can only provide high-acuity information about colour in bright environments. Rods, do not distinguish colour and are more sensitive to light. For instance, in 1866, researchers observed that nocturnal species have rod-only vision, and diurnal species have cone-only vision. These mechanisms present an important point about the nature of vision, with reference to the duplexity theory of vision, because it displays a shift ability respective to relative lighting in the environment, allowing us as humans to see at all times of a 24-hour day.
Module 7 Question: Attention in Various Patient Populations
Attention is a universal component of brain and behaviour mechanisms in the human brain. Although difficult to define, attention can be more or less defined as perception and intake of our surroundings. Moreover, selective attention is defined in two ways. One, our perception of the surrounding environment is improved when we focus on something. Two, any stimulus that is not in focus is interfered. The prefrontal and posterior parietal association cortices are implicated in during selective attention processes, where it is believed that neural responses are strengthened with focus on stimulus; increasing cortex stimuli and conversely, weakened by another stimulus.
External events and internal cognitive processes play a critical role in attention. For instance, endogenous attention is catalyzed by internal cognitive process, such as cleaning your house because it is dirty. On the other hand, exogenous attention is mediated from lower to higher neural mechanisms elicited by external events such as doorbell going off. Another type of attention is space-based attention where, like mentioned above, what the visual system focuses on in the surrounding environment is a stimulus and hence, strengthens neural responses to what is in focus.
Throughout different patient populations, attention and attention related disorders becomes relevant, as a couple examples of space-based attention disorders are brought to light. Patients with damage to the right parietal lobe display a space-based attention disorder known as contralateral neglect. Contralateral neglect is defined as “A disturbance of the patient’s ability to respond to stimuli on the side of the body opposite to a site of brain damage…” (Barnes, Pinel 2017) This disorder aided in our understanding of the attention because it displays that attention is a mechanism of the brain as a whole, and when one area of the brain is lesioned, it in turn effects our understanding of our surroundings and ability to focus on presented stimulus.
The inability to consciously perceive the left side of space (egocentric) and objects is baffling. However, the contralateral neglect patient population provides us with opportunity to explore and grow to understand brain and behaviour mechanisms related to attention. (Fierro 2000) The right side of the brain is not able to focus on, perceive or receive information from the left side of space, further portraying the notion that attention is not one component, but a sum of the parts of the brain.
Module 8 Question: How You Move
The sensorimotor system is a system that is organized hierarchically, where higher levels in this functional segregation carry out more complex functions. In general, sensory inputs guide motor outputs, requiring constant sensory feedback from the relative motor movements, such as the feeding response. The primary motor cortex is one of many components part of the sensorimotor system, in the gyrus located in the frontal lobe of the brain. Over the last 20 years, the understanding of the organization and functions of the primary motor cortex have changed dramatically. In comparison, it has become evident that the current knowledge of the organization and functions of the primary motor cortex are built upon traditional view.
The way the sensorimotor system is organized plays an important role, as one of the functional areas of the cerebral cortex, the primary motor cortex releases motor signals that descend into lower levels of the sensorimotor system. In 1937, the primary motor cortex was organized somatotopically where various parts of the body are displayed on the motor homunculus. Through neurosurgery and short applications of electrical stimulus on the cerebral cortex, Penfield and Boldrey put together the motor homunculus by observing, with each stimulation, which body part moved. Despite some overlapping, this work brought to light the fact that the primary motor cortex is responsible to complex motor movements. Conventionally, the primary motor cortex neurons coded for their selected direction of movement; conversely, this may not be the case.
Recent testing on monkeys disputed the latter idea. Research presented that primary motor cortex neurons do not code for their preferred pathway, rather, these motor neurons encode for a specific end location. After training the subjects to make precise arm movements, researchers found that when the movements were made from a particular angle, neurons fired. (Hari 1998) Afterwards, neuronal activity was analyzed without control disproving the notion that primary motor cortex neuronal activity follows a direction of movement. Therefore, stimulations of the primary motor cortex contained in an action map, shows that different muscles and their respective locations can have the same end point. The current view is that the primary motor cortex is involved in multifaceted species-typical motor movements, where more body parts are involved and the somatotopic organization (Penflied) falls short. In 2015, Grazino and other researchers re-mapped out the primary motor cortex through longer electrical currents of 0.5-1.0 seconds, showing that Penfield’s motor homunculus needed to be revisited. For instance, an example of a more complex motor movement is drinking a cup of coffee, one must make a series of movements to grab and pick-up the mug, bringing it to your mouth.
The past two decades have represented a great deal of growth in understanding the organization and functions of the primary motor cortex. Not only do we now know a lot more information about the cerebral cortex and its functional areas, we can observe that research is important because it builds upon our foundations of knowledge, challenging perceptions and enhancing understanding.