- as people get older it generally takes them longer to react to a GO signal and to actually complete the movement.

- in everyday life this means that you can't do as many tasks in a day.

- also means that you will be slower in responding when driving a car, recovering your balance after a slip, etc.

- these temporal aspects of movement can be broken down into "reaction time" and "movement time".

- Definition: duration from the onset or appearance of a stimulus to the initiation of a response to that stimulus.

- Implication: delays in one or several of the steps by which a physical stimulus (e.g., a light) is transformed into neural activity and subsequently used to elicit a motor response.

- Different types of reaction time: simple reaction time, choice reaction time.

    - Simple reaction time (SRT): respond to a stimulus with simple movement, SRT slows by 0.6ms/year from 20 to 96 years.

    - Choice reaction time (CRT):  - make a specific movement for a specific stimulus.
                                                   - for example, green light - flex index finger; red - extend.
                                                   - have to i.d. stimulus, decide which response to select, and initiate movement.

    - with aging get a disproportionate increase in the time required to decide about stimulus-response coupling in CRT tasks.
 
    - also, the effect of increases in response complexity on reaction time are exaggerated in older adults.

- Reaction times can also be "fractionated" or divided into different components representing different parts of the process by which the sensory stimulus is transformed into the motor response.

- this can be examined by measuring the visual and motor evoked potentials using electroencephalography (EEG) and measuring the onset of muscle activity using electromyography (EMG).

- the onset of the visually-evoked potential provides an index of when the visual stimulus reaches the visual cortex.

    - in older adults the latency from the onset of the stimulus to the onset of the VEP is lengthened.

- time from onset of VEP to onset of motor-evoked potential provides information about the central processing time.

    - in older adults this duration is lengthened, especially in CRT tasks - this is probably the main source of slowness.

- time from onset of motor-evoked potential to onset of EMG tells you about nerve conduction velocity.

    - slightly longer duration for this component - corticospinal neurons do not transmit their messages as efficiently.

- time from onset of EMG to onset of movement is an index of ability of muscles to generate sufficient force to move limb.

    - only slightly delayed in older adults - related to decrements in muscle function.

- Definition: time required to complete a movement.

- Implication: the processes that contribute to the control of the movement as it occurs are degraded in older adults.

- related to ability to produce sufficient contractile strength in the appropriate muscles - both agonists and antagonists.

- in even simple movement tasks like tapping there is an increase in movement time.

- when the task is made more difficult (reciprocally tapping two targets) the age-related increases in movement time are exaggerrated.

- the ability to activate particular groups of muscles in the appropriate order to invoke smooth, efficient movement of several body parts.

- for coordinated movements to occur need to have a close interaction between the perceptions and actions involved with the task. For example, to reach for or point at an object need to accurately locate it in space, and decide its size, shape, orientation, etc. These physical attributes can then be transformed into appropriate motor commands to produce a reach and grasp response.

- as with the slowing of reaction time the breakdown in coordination observed in aging adults can be attributed to less efficient processing within one or several of these processes.

- can see effects of aging on reaction times and movement times for discrete movements.

- can also assess the abilities of aging adults to use prior information to help in preparing discrete movements.

- providing information about which arm to use, which direction to go, and by how much you can completely "preprogram" the response - if then give a different cue for the GO signal the subject has "reprogram" response.

- older adults take more time to reprogram movements that require a change of arm or arm and direction.

- aging adults react and move slower, but does this make them more or less accurate in getting to the target?

- in reciprocal tapping tasks (Fitts paradigm) older adults display a larger increase in movement time as the accuracy demands increase compared to younger adults.

- thus, to achieve the same levels of accuracy older adults have to move slower.

- older adults do this by taking relatively longer during the deceleration or "homing in" phase of the movement during which the hand approaches the target.

- this may be the result of a breakdown in the coordination between the agonist and antagonist muscles involved in the movement - in older adults the antagonist sometimes does not become activated soon enough during the response to "brake" the movement.

- movement is constantly adjusted to follow the changes in position of the target - e.g., smooth pursuit eye movements.

- steering a car and handwriting are real world examples of continuous movements.

- in the lab one can test similar types of movements under well-controlled conditions - tracking tasks.

- in general, older adults are less accurate when attempting to track a continuously moving target.

- under these conditions, the target determines how fast they must move - this suggests that when young and old adults move at the same speed, older adults are less accurate.

- as for real world tasks, it has been shown that the time taken to write either single digits, entire words, or trace figures increases with age.
 

- Saccades - like discrete aiming movements - RT and MT increase, peak velocity decreases, accuracy stays the same

- Smooth pursuit - like continuous tracking movements - RT unchanged, gain decreased, saccadic corrective movements increased.

- since older adults appear to be at a lower level of motor performance it may be possible for them to show greater amounts of motor learning.

- there is some evidence that this is the case - may be due to more consistent performance within each older subject in a particular study.

- despite these improvements there is still an age difference on most motor tasks after extensive practice.

- such improvements support the notion that experience-dependent plasticity is a process that can still take place in an old brain.