Visual pathways in the brain, and what happens when they bre...
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Visual pathways in the brain, and what happens when they bre...
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[post_date] => 2024-12-25 13:45:35
[post_date_gmt] => 2024-12-25 18:45:35
[post_content] => Practice Passage (Question 1-5)
*This passage is the property of Khan Academy and has been reformatted into an AAMC-style interface in their entirety by MedLife Mastery. MedLife Mastery does not endorse and is not an affiliate of Khan Academy.
Each of our senses follows the same basic neural pathway to transmit information to the brain. First, energy is converted into a neural impulse. Then, those impulses are transmitted along receptor neurons that transduce the information for the brain. Finally, the receptors send the signal to the cerebral cortex which interprets the information. The visual pathway begins with light rays passing through the cornea and centering at the fovea and back of the retina. When the light rays focus at the back of the eye, they are processed by specialized cells known as rods and cones. These photoreceptors transduce the photons into action potentials to be passed along the optic nerve to eventually reach the thalamus and then the cerebral cortex.
Once these impulses reach the occipital lobe of the cerebral cortex, the brain must manage and sort the information. Each eye sees a different perspective of the image, and thus sends different information along its optic nerve. The cortex is responsible for creating a single, stable image from this data, and then processing it. Visual analysis occurs through two principal paths – the dorsal “where” pathway, and the ventral “what” pathway. The former pathway runs to the parietal lobe of the brain, while the latter leads to the temporal lobe.
When either of these pathways fails, there are consequences for the way we perceive our world. One such disorder is known as neglect syndrome. When asked to redraw a picture, patients copy only one side of the model, while ignoring the other part. On the other hand, patients with visual agnosia can draw the whole object, but they cannot recognize what they have created. Though they see the image, it does not translate into an idea in their minds.
Two patients with neural damage, Angela and Zelda, were asked to perform a series of tasks designed to test the visual pathways in the brain. In the first task (Figure 1), each was shown pairs of faces and asked to determine whether they were identical. In the second task (Figure 2), Angela and Zelda were asked to trace a series of pictures onto transparent paper.
Figure 1. Discrimination between identical and different faces. Subjects were shown pairs of faces and asked to discriminate. In the first trial, faces were oriented in the same direction. The following trial varied the orientation of the faces. Results shown as percent correct of 50 trials.
Figure 2. Tracing test. Patients were asked to trace over pictures on transparent paper. The first set of trials used pictures of known objects, and the second trial asked patients to trace abstract lines. Data shown as percentage traced in 3 minutes.
Data adapted from:
Goodale, M. (1994). Separate neural pathways for the visual analysis of object shape in perception and prehension. Current Biology, 604-610
[post_title] => Visual pathways in the brain, and what happens when they break
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[question] => A patient was asked to copy a picture of a clock and produced the following image:
Doctors immediately suspected a lesion in the right hemisphere. What does this imply about optic nerves organization?
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[answer] => 1
[description] => Reason for the Correct Answer:
There is a set of optic nerves coming from each eye, where they intersect is known as the optic chiasm
When you close one eye, you see an image similar to the one you see when both eyes are open.
The right half of the field of view is processed by the left side of the cerebral cortex, and vice versa, so a lesion in the right hemisphere would disrupt processing of the left field of view.
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[answers] => Array
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[0] => Array
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[each_answer] => A. The right half of the image from both eyes is transmitted to the left hemisphere of the visual cortex, and vice-versa
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[1] => Array
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[each_answer] => B. The optic nerves from the left eye go to the right hemisphere of the cerebral cortex, and vice-versa
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[each_answer] => C. The optic nerves from the left eye go to the left hemisphere of the cerebral cortex, and the right eye connects to the right hemisphere
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[each_answer] => D. The right half of the image from both eyes is transmitted to the right hemisphere of the visual cortex, and the left half of the field of view transmits to the left hemisphere
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[1] => Array
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[quiz_unique_key] => 3873426850
[question] => Visual agnosia is a disorder of which pathway?
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[answer] => 2
[description] => Reason for the Correct Answer:
The temporal lobes are responsible for processing auditory signals, interpreting visual stimuli, and language recognition.
The parietal lobes are responsible for spatial reasoning and receiving somatosensory information.
Visual agnosia is a disorder of the ventral pathway, because it is an inability to recognize an image.
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[answers] => Array
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[0] => Array
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[each_answer] => A. The dorsal pathway, because it is a disorder of spatial attention
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[each_answer] => B. The ventral pathway, because it is a disorder of object perception
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[2] => Array
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[each_answer] => C. The dorsal pathway, because it is a disorder of object perception
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[each_answer] => D. The ventral pathway, because it is a disorder of spatial attention
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[quiz_unique_key] => 83407773
[question] => Zelda most likely has neural damage in which pathway?
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[answer] => 4
[description] => Reason for the Correct Answer:
The parietal lobes are responsible for spatial reasoning and receiving somatosensory information.
Zelda was able to recognize faces, but performed more poorly when asked to recognize an entire object.
Zelda most likely has damage in the dorsal pathway, as she has difficulty with spatial awareness, but not with object perception.
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[0] => Array
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[each_answer] => A. The dorsal pathway, because she has trouble with object perception
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[1] => Array
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[each_answer] => B. The ventral pathway, because she has trouble with spatial attention
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[each_answer] => C. The ventral pathway, because she has trouble with object perception
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[each_answer] => D. The dorsal pathway, because she has trouble with spatial attention
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[quiz_unique_key] => 2261298308
[question] => Angela’s doctor suspects she has damage in her temporal lobe. Which of the following tests would be the least useful in testing the hypothesis?
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[answer] => 2
[description] => Reason for the Correct Answer:
The temporal lobes are responsible for processing auditory signals, interpreting visual stimuli, and language recognition.
Damage to the temporal lobe could mean that Angela has difficulty connecting the visual perception of an object with the rest of her knowledge about that object.
Angela would be able to color in a coloring book regardless of temporal lobe damage, because the task does not require her to recognize what the object is.
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[answers] => Array
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[0] => Array
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[each_answer] => A. Asking Angela to read from her favorite book
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[1] => Array
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[each_answer] => B. Asking Angela to color a page in a coloring book
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[2] => Array
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[each_answer] => C. Asking Angela to pick her mother out in a family photo
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[3] => Array
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[each_answer] => D. Asking Angela to describe her favorite painting
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[quiz_unique_key] => 574431310
[question] => Sound travels along a similar path to light. Which portion of the auditory pathway most closely corresponds to the rods and cones?
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[answer] => 2
[description] => Reason for the Correct Answer:
Rods and cones are responsible for transduction, the process in which a stimulus is changed from one form to another to send it to the nervous system.
Sound travels through the ear in mechanical waves, causing the eardrum to vibrate.
Though the basilar membrane does vibrate with the incoming sound waves, it is the hair cells in the inner ear that transduce the signal and send it along the auditory nerve.
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[answers] => Array
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[0] => Array
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[each_answer] => A. Outer ear
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[1] => Array
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[each_answer] => B. Hair cells
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[each_answer] => C. Basilar membrane
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[each_answer] => D. Eardrum
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[558683|3] => D
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