Working of JFET as amplifier
It's an input protection circuit.1
In normal operation, both JFETs are conducting, and the input signal flows through R1.
However, if the voltage across R1 begins to approach the threshold voltage of either JFET2, the JFET will start to cut off, restricting the current and limiting the voltage that is developed across Rt1, and therefore limiting the voltage into OPA277.

1 Note that to a certain extent, it also helps to protect the source of the brain waves from internal faults in the equipment.
2 On the order of 0.7 V for the 2N4338, for a current limit of about ±100 µA.