Lec 8 | MIT 9.00SC Introduction to Psychology, Spring 2011
MIT OpenCourseWare・44 minutes read
The lecture explores human attention limitations and introduces attention disorders like blindsight and neglect, demonstrating how some individuals subconsciously perceive visual stimuli despite lacking conscious awareness. Through various experiments and case studies, it illustrates the complexities of these disorders, revealing that unconscious processing can still occur in the presence of significant brain damage.
Insights
- The lecture highlights the limitations of human attention, illustrating how much information we miss in our environment, with some stimuli entering our awareness unconsciously, as demonstrated through the conditions of blindsight and neglect.
- Blindsight is a fascinating phenomenon where individuals, despite being cortically blind, can respond to visual stimuli without conscious awareness, suggesting that the brain has an unconscious processing system, particularly involving the superior colliculus.
- The text discusses neglect, a disorder where individuals fail to acknowledge one side of their environment, as exemplified by a woman who only eats from the right side of her plate after a stroke, showcasing the profound impact of neurological injuries on spatial awareness.
- Experimental findings reveal that even when patients with neglect do not consciously perceive objects on their neglected side, their brains still process some information, as evidenced by their ability to report differences in objects when they are not identical, indicating a complex interplay between awareness and unconscious processing.
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Recent questions
What is visual neglect?
Visual neglect is a neurological condition where individuals fail to notice objects or stimuli on one side of their visual field, typically the left side, due to damage in the right hemisphere of the brain. This condition often arises after strokes or brain injuries and is characterized by a lack of awareness of the neglected side, leading to significant challenges in daily activities. Patients may demonstrate this neglect by ignoring the left half of their environment, such as only eating from the right side of a plate or failing to acknowledge items on the left when asked to report what they see. The phenomenon of "extinction" can also occur, where patients can perceive stimuli on both sides when presented separately but struggle to notice those on the neglected side when both sides are shown simultaneously. This condition highlights the complexities of spatial awareness and cognitive processing in the brain, revealing how neurological impairments can profoundly affect perception and behavior.
How does blindsight work?
Blindsight is a fascinating phenomenon where individuals who are cortically blind due to damage in the primary visual cortex can still respond to visual stimuli without conscious awareness. This suggests that there are alternative pathways in the brain that process visual information outside of conscious perception. For instance, the superior colliculus, part of the brain involved in reflexive eye movements, plays a crucial role in this unconscious visual processing. Patients with blindsight can accurately guess the location or characteristics of objects in their blind field, even though they claim to see nothing. Historical experiments with animals, such as dogs with bilateral occipital lesions, demonstrated that they could navigate their environment despite being cortically blind, indicating that some level of visual processing occurs unconsciously. This phenomenon challenges our understanding of perception and consciousness, revealing that the brain can still process visual information even when individuals are not consciously aware of it.
What is anosognosia?
Anosognosia is a neurological condition in which individuals are unaware of their own disabilities or impairments, often resulting from brain injuries or strokes. Patients with anosognosia may deny having a problem, even when it is evident to others. For example, a patient with paralysis on one side of the body might insist that their affected limb is functioning normally. This disconnect arises because the brain injury impairs the patient's ability to construct a reality that acknowledges their condition, despite their cognitive capacity to understand it. Anosognosia can complicate rehabilitation efforts, as patients may not recognize the need for treatment or assistance. The condition highlights the intricate relationship between self-awareness and neurological function, illustrating how brain damage can lead to profound gaps in a person's understanding of their own health and capabilities.
How do infants develop visual attention?
Infants develop visual attention through a gradual maturation of their visual processing systems, which is evident in their eye movements and responses to stimuli. Research has shown that even very young infants can identify and respond to visual stimuli in both their "good" and "blind" visual fields. As they grow, particularly between 8 to 12 weeks of age, their ability to focus and compete for visual attention becomes more refined. In experiments, infants consistently moved their eyes toward stimuli in both fields when there was no competition for attention. However, when a fixation dot was introduced, their likelihood of noticing stimuli in the blind field decreased, indicating that as their cortical systems mature, their conscious attention becomes more selective. This developmental shift suggests that the conscious visual processing capabilities of infants evolve over time, leading to a more sophisticated understanding of their visual environment as they grow.
What is the role of the superior colliculus?
The superior colliculus is a critical structure in the brain that plays a significant role in visual processing and attention, particularly in the context of reflexive eye movements and unconscious visual perception. It is involved in directing gaze and coordinating eye movements toward stimuli in the environment, even when individuals are not consciously aware of those stimuli. In cases of blindsight, for instance, the superior colliculus allows individuals to respond to visual cues in their blind field, suggesting that it processes visual information outside of conscious awareness. This structure connects the retina to various brain regions, facilitating the rapid processing of visual information and enabling quick reflexive responses. The superior colliculus thus serves as an essential component of the visual pathway, highlighting the complexity of how the brain integrates sensory information and directs attention, even in the absence of conscious perception.
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Summary
00:00
Unconscious Visual Processing and Attention Disorders
- The lecture discusses the limitations of human attention, emphasizing that we are often unaware of the vast amount of information in our environment, with some information sneaking in unconsciously.
- Two significant disorders of attention are introduced: blindsight, where individuals are cortically blind yet can perceive some stimuli, and neglect, which affects spatial awareness and perception.
- Blindsight is characterized by the ability to respond to visual stimuli without conscious awareness, suggesting the existence of an unconscious visual processing system in the brain, particularly involving the superior colliculus.
- The visual pathway is explained: information from the left visual field is processed in the right primary visual cortex, and vice versa, with a smaller pathway (tectopulvinar system) connecting the retina to the superior colliculus.
- Historical experiments on dogs with bilateral occipital lesions demonstrated that these animals could navigate their environment despite being cortically blind, indicating some level of unconscious visual processing.
- A case study of a human patient with a large injury in the right visual cortex showed that he was blind in the left visual field but could still respond to stimuli in that field, demonstrating blindsight through perimetry tests.
- The patient’s responses were measured using a computer monitor, revealing that he could accurately guess the length of lines presented in his blind field, despite claiming he saw nothing.
- Another patient who suffered two strokes resulting in blindness in both visual fields exhibited similar behavior, reinforcing the idea that the brain processes visual information unconsciously.
- Experiments with monkeys that had unilateral lesions showed that they could accurately respond to stimuli in their blind field when forced to point, yet they reported seeing nothing, mirroring human experiences of blindsight.
- Infants who underwent hemispherectomies for severe medical conditions also displayed similar patterns of unconscious visual processing, further supporting the existence of non-conscious visual pathways in the brain.
12:41
Infant Visual Development and Attention Disorders
- The experiment investigates the development of the conscious visual system in infants, focusing on how their eye movements indicate visual processing capabilities during their first year of life, particularly at ages 8, 10, and 12 weeks.
- Researchers recorded the eye movements of infants when presented with stimuli in both their "good" visual field and "blind" field, using a fixation dot to measure attention and competition for visual focus.
- In the non-competition condition, infants consistently moved their eyes to stimuli in both fields, indicating an ability to identify and respond to visual stimuli regardless of their location.
- In the competition condition, the presence of a fixation dot reduced the likelihood of infants noticing stimuli in the blind field, suggesting that as their cortical system matures, their conscious attention becomes more selective.
- The study highlights that as infants develop, their conscious visual processing suppresses the ability to respond to stimuli in the blind field, indicating a shift in how their visual system operates with increasing maturity.
- The text transitions to discussing attention and neglect disorders, particularly focusing on patients with neglect following strokes or brain injuries, which are common in clinical settings.
- An example is provided of a woman in her 60s who, after a massive stroke affecting her right hemisphere, only eats from the right half of her plate and ignores the left side entirely, demonstrating a failure to acknowledge the left visual field.
- The phenomenon of "extinction" is described, where patients can see stimuli on both sides when presented separately but fail to notice stimuli on the neglected side when both sides are presented simultaneously.
- Anosagnosia is introduced as a condition where patients deny any problems with their neglected side, illustrated through a case where a patient with a right hemisphere stroke insists her left hand is functioning despite it being paralyzed.
- The text concludes with experimental examples demonstrating the disconnect between the patients' perceptions and their actual physical capabilities, such as failing to hold a tray properly or acknowledging sensations in their neglected side, emphasizing the complexity of these neurological conditions.
24:43
Understanding Anosognosia and Visual Neglect
- The narrative begins with a patient experiencing paralysis in her arm, which she identifies as her own but cannot use, indicating a neurological issue. The doctor questions how long the paralysis has been present, revealing that the patient has been aware of it for several days.
- After approximately 90 minutes of treatment involving irrigation to the ear, the patient's condition temporarily improves, highlighting the brain's ability to recover from certain disorders over time, unlike more severe conditions such as Balint's syndrome or spatial disorders.
- The text discusses anosognosia, a condition where patients deny their illness, emphasizing that their brain injury prevents them from constructing a reality that acknowledges their disorder, despite having the cognitive capacity to understand it.
- Examples of neglect are provided, particularly in patients with right-sided lesions who exhibit a lack of awareness of the left half of their environment, demonstrated through various tests such as crossing out lines or identifying letters.
- A cancellation test shows that patients often neglect the left side of a visual field, failing to mark or cross out items on that side, indicating a profound lack of awareness rather than a simple visual impairment.
- The text describes how patients with visual neglect may read or copy objects, often omitting the left side entirely, as illustrated by a patient reading a text and only acknowledging the right half of the page.
- Clocks are used as a common test for neglect, where patients can only draw the right side of the clock face, demonstrating their inability to perceive the left half, even when they know the numbers should be evenly distributed.
- The narrative explains that neglect can occur across different modalities, affecting not just vision but also actions like reaching for objects, indicating a broader cognitive impairment related to spatial awareness.
- The location of brain damage associated with neglect is debated, with recent findings suggesting that the temporal lobe plays a crucial role in attention balance between the left and right parietal cortices, rather than the parietal cortex alone.
- Finally, experiments reveal that patients with neglect also struggle to imagine familiar places, such as the Piazza del Duomo in Milan, demonstrating that their cognitive neglect extends beyond immediate perception to their mental imagery and memory.
35:54
Cognitive Bias and Left-Sided Neglect Explained
- The text describes a scenario involving a square with various locations, where individuals are asked to report what they see from different perspectives, specifically from the church's viewpoint, leading to a phenomenon known as left-sided neglect.
- Participants consistently ignore items located on the left side of the square when prompted to recall what they see, demonstrating a cognitive bias that prevents them from acknowledging the left half of their visual field.
- This neglect is likened to a neurological condition where patients are unaware of their inability to perceive the left side, akin to someone not recognizing a problem with their own arm.
- An experiment is introduced where patients are shown clouds through a narrow slit, requiring them to compare two clouds; however, they struggle to identify differences on the left side due to their neglect.
- A simple task is described where participants push a button when a light appears, with varying conditions of arrows indicating where the light will come on, revealing how attention is affected by misleading cues.
- The experiment shows that patients with left-sided neglect perform poorly when an arrow directs their attention to the good side, indicating difficulty disengaging from that side to attend to the neglected side.
- The process of shifting attention involves three steps: disengaging from the current focus, moving attention to a new location, and then focusing on that new location, which is particularly challenging for patients with neglect.
- Drawing a clock is used as an example; patients often fail to draw the left half of the clock when starting from the top, but can accurately draw specific times like 8 o'clock when not prompted to focus on the good side.
- The text discusses an experiment where a display is rotated in front of patients, demonstrating that their neglect can shift with the display, indicating that their perception of left and right is determined by their mental assignment rather than absolute spatial orientation.
- Lastly, patients are asked to choose between two similar houses, revealing that while they may not notice differences, they can still express a preference, suggesting that some level of processing occurs outside of conscious attention.
47:06
Understanding Visual Neglect and Awareness
- The text discusses the concept of visual neglect, where individuals with right-sided lesions may fail to report objects on their left side, particularly when presented with identical items, such as two forks. This phenomenon suggests that even when they consciously ignore an object, their mind still processes information about it, indicating a level of awareness that influences their reporting behavior based on the similarity of objects. For example, if presented with a fork and a spoon, they may report the spoon if they recognize it as different, but may overlook the left-side fork if it is identical to the right-side fork.
- A video featuring neurologist Bob Rafal and a patient illustrates this concept, showing that the patient often fails to notice identical objects on the neglected side, reporting only one item when they are the same. However, when different objects are presented, the patient tends to report the one on the neglected side, albeit more slowly. This behavior highlights that the patient's mind retains some knowledge of the objects, allowing them to differentiate and report based on their awareness, even if they consciously neglect the information.
