Reflexes and Reaction Times

Overview

During our day-to-day lives we detect changes in the environment and react appropriately.  An external stimulus is detected by one or more neurons or specialized sensory receptors, which send the sensory information to the central nervous system (CNS) where it’s processed.  If a motor response is initiated, it usually involves a series of action potentials (APs) that produce muscle contraction(s) and the movement of one or more parts of the body.  A simple reflex is perhaps the easiest of this type of stimulus-response pathway.  A loud sound or something flying at your eye makes you blink, while a tap on the tendon under the knee cap produces the knee-jerk (myotactic) reflex. 

A simple reflex like the myotactic reflex is produced via single synapses between sensory axons and motor neurons.  The required circuitry for this reflex is confined to the spinal cord.  While sensory information may ascend to higher centers, the brain is not necessary or required to perform the reflex.  More complex reflexes usually involve additional (inter-) neurons and more than one population of motor neurons.  Thus, more neurons and synapses are involved, which usually results in a longer delay between stimulus and response and often a more complex response. 

In this lab you will study the time taken between a stimulus and response.  These reaction time measurements will be made from a human volunteer subjected to harmless audio and visual stimuli.  The exercise should allow you to make conclusions regarding the relative complexity of these sensory systems as well as the role priming and prediction play in altering responses to these types of stimuli.   In the second part of today’s exercise, you will elicit the myotactic reflex from a human subject and determine the extent to which the brain exerts inhibitory control over this reflex. 

 

            

: A cross section of the spinal cord showing the single synapse between the sensory and the motor neurons involved in the myotactic reflex.

Equipment Required

PC computer

IWorx/214 and USB cable

Event marker

Plethysmograph

Equipment Setup

1.       Plug the DIN connector of the plethysmograph (pulse transducer) into Channel 3 of the iWorx unit (see figure).

2.       Plug the DIN connector of the event marker into Channel 4 of the iWorx unit.

         

: The equipment used to measure reaction times from a volunteer.

Start the Software

1.       Click the Windows Start menu, move the cursor to Programs and then to the iWorx folder and select LabScribe; or click on the LabScribe icon on the Desktop.

2.       When the program opens, select Load Group from the Settings menu.

3.       When the dialog box appears, select Ak214.iws and then click Load.

4.       Click on the Settings menu again and select the Membrane 3 settings file.

5.       After a short time, LabScribe will appear on the computer screen as configured by the Reflex-Reaction-214 settings.

·          Note the sampling speed and display time and record these values in your laboratory notebook.

 


Exercise 1: Reaction Time and Sound

Goal: To measure the reaction time to an audio stimulus. 

Procedure

1.       Choose a member of your group to act as a subject for the next set of (non-invasive) experiments.

2.       Seat the subject in a chair placed so that the subject’s back is to the computer screen and the keyboard.  Ask the subject to close his/her eyes to ensure the computer monitor cannot be seen.  Ask the volunteer to relax.

3.       Have the subject listen as another student taps the white surface of the plethysmograph with a pencil. Make sure the subject CAN HEAR the tapping sound but CANNOT SEE the individual tapping the pulse transducer!

4.       Ask the subject to click the event marker as soon as they hear the tap.

5.       Click Start.

6.       Present the subject with a total of 10 taps; but deliver the taps in an IRREGULAR PATTERN that is difficult for the subject to predict.

7.       Click Stop to halt recording after 10 responses have been recorded.

8.       Select Save As in the File menu and save the file within the Section Folder (ex: Monday Lab) within My Documents. Click the Save button to save the file (as an .iwd file).

Data Analysis

1.       Scroll through the data until you locate the first stimulus-response event.

2.       Click the 2-Cursor icon, so that two blue vertical lines appear over the recording windows.

3.       Drag the cursors apart so that the large spike on the plethysmograph (Ch3) and the signal from the event marker (Ch4) are located between the two blue lines.

4.       Click the Analysis icon to open the Analysis window or select Analysis from the Windows menu.

                 

: The LabScribe toolbar

5.       Display Channels 3 and 4 by de-selecting Channels 1 and 2 in the Display Channel list, on the left side of the Analysis window.  Select Title and T2-T1 from the Table Functions list.

6.       Use the mouse to click and drag one cursor to the beginning (BE PRECISE!) of the spike on the Stimulus channel (Ch3) and the second cursor to the onset of the response signal from the event marker (Ch4). 

7.       Copy and paste your trace and the corresponding T2 – T1 value into the Journal.  Data can be entered into the Journal by either typing the titles and values directly or by using the right-click menu (select Add Title to Journal or Add Data to Journal from the right click menu).  Print.  Record the data in your laboratory notebook.

8.       Return to the Analysis window by selecting the Analysis icon from the toolbar or by selecting Analysis from the Windows menu.  Use the scroll bars to move through your data and repeat the measurements for all 10 trials (do not paste these values in the Journal & do not print!).  If the subject did not respond to a stimulus, record it as NO RESPONSE in your lab notebook. Record the other values in your laboratory notebook.

9.       Drop the longest and shortest times from the data set (a NO RESPONSE does not count as the shortest reaction time) and average the remaining eight values to determine the mean reaction time.  If the subject did not respond to a stimulus, drop it AND the shortest and longest times before calculating the average for the remaining values. Record in your lab notebook.

 

                     

 

: Data produced by tapping the plethysmograph, which entered a large spike on the lower trace and produced a sound, which the volunteer used as a cue to press the event marker (upper trace). The data are displayed in the Analysis window and the two cursors are positioned to measure the reaction time (T2-T1).

 

Exercise 2: Reaction Time and Prompted Sounds

Goal: To measure the reaction time to sounds delivered immediately after a verbal prompt.

 Procedure

1.     Begin a new trace by selecting New under the File pull down menu.  You do not need to save the trace from Exercise 1.

 

2.    Repeat Exercise #1, but prepare the subject by giving him/her a verbal cue immediately prior to each tap on the plethysmograph.

Data Analysis

Measure the time interval (T2-T1) between the onset of stimulus and response for each event and record these values in your laboratory notebook.  Drop the longest and shortest times from the data set and average the remaining values to determine the mean reaction time.  Do NOT include data where the subject responded BEFORE the onset of a stimulus!  If the subject responds before 3 or more stimuli, you should repeat the exercise.  Record all values in your lab notebook.

 


Exercise 3: Reaction Time and Predictable Sounds

Goal: To measure the reaction time to sounds delivered at a predictable interval.

Procedure

Repeat Exercise #1, but tap the plethysmograph at a predictable interval.  Do NOT prompt the subject with an audio cue!

Data Analysis

Measure the time interval (T2-T1) between the onset of the stimulus and response for each event and record these values in your laboratory notebook.  Drop the longest and shortest times from the data set and average the remaining values to determine the mean reaction time.  Do not include data where the subject responded BEFORE the onset of a stimulus.  If the subject responded before 3 or more stimuli, you should repeat the exercise.  Record all values in your lab notebook.

 

Questions

1.             Is the average reaction time the same for the three conditions above (exercises 1 – 3)?

2.             Which condition had the longest average reaction time? The shortest?  Why?

 

Exercise 4: Reaction Time and Visual Cues

Goal: To measure the reaction time to a visual cue. 

Procedure

1.       Select New under the File menu to begin a new trace.

2.       Have the subject (same subject as in the previous exercises) sit in a chair and face the computer screen.

3.       A second student should stand out of sight of the subject to gently and quietly tap the plethysmograph.  Make sure the subject CANNOT HEAR the tapping on the plethysmograph.  Also make sure the subject does NOT SEE the tapping directly.   

4.       Instruct the subject to watch the computer monitor and press the event marker when a deflection in the stimulus trace becomes visible.

5.       Click Start. Present the subject with a total of 10 trials, delivered in a pattern that is UNPREDICTABLE.

6.       Click Stop to halt recording after 10 responses have been recorded.

7.       Select Save As from the File menu and save the file under the Section Folder within My Documents.

Data Analysis

1.       Use the cursors to measure the time delay between the first visual stimulus (the onset of the tap; deflection on Ch 3) and the response (the onset of the deflection in Ch 4). 

2.       Repeat the measurement for all 10 trials.

3.       Drop the longest and shortest times from the data set, and average the remaining eight values to determine the mean reaction time.  Record this value in your notebook.

 

                  

Data produced using a visual cue.  The volunteer responded to a visual cue (lower trace) by pressing on the Event Marker (upper trace).  The data are displayed in the Analysis window and the two cursors are positioned to measure the reaction time (T2-T1).

Questions

1.      How does the average reaction time from this exercise compare to the data from Exercise #1? Why might the reaction time to a visual stimulus differ from the reaction time to an auditory stimulus? Explain.

 

 


Exercise 5: Reflexes

Aim: To examine the myotactic (knee-jerk) reflex and investigate conscious control over the reflex.

The Knee-Jerk Reflex

1.       Choose a member of your group to act as a subject.  Ask the subject to sit in a chair and cross his/her legs.

2.       Have another member of the group use a percussion hammer to firmly strike the subject’s tendon below the knee-cap.  Record observations of the subject’s response in your notebook.

3.       Ask the subject to count from 100 to 1 (backwards) and repeat step #2 above. If the response differs from #2, be sure to describe it in your notebook. 

4.       Next, have the subject curl their fingers toward the palm of each hand to form a cup. The subject should interlock their hands so the fingers of one hand fit in the "cup” of the other.  Ask the subject to hold their arms in front of their chest with their elbows pointed out and pull their hands in opposing directions (the subject ‘s hands should stay interlocked during the exercise). This is the Jendrassik maneuver; it develops motor activity in the arm and shoulder muscles.

5.       While the subject is engaged in the Jendrassik maneuver, repeat step #2 and observe the subject’s response in the presence of motor activity in another part of the body.  Record your observations, particularly any differences you notice between the responses to the conditions above. 

 

Questions 

1.       What effect does mental activity have on the knee-jerk reflex?

 

2.       What effect does the Jendrassik maneuver have on the knee-jerk reflex?

 

3.       What conclusions can you make regarding conscious control over this reflex?